If you're an author, and you’ve got publishers and/or readers who trust you, you can just go on and on with the one story, if you want. As far as physical copies of books go, there is a limit with how many pages can be bound together and sold conveniently, but a story can be split into volumes or parts (and then possibly never finished… oh, hey, A Song of Ice and Fire), or if it’s an eBook, it can be pretty much limitless.
The best epic books of all time, though? Yes, quantity (of pages) plays a role in such books being considered epics, but quality also has to be taken into account. These books have quantity and quality, so to speak. They're excellent epics that are worthy of devoting lots of your time to (most of them are standalone books, though there have been some multipart stories included, too).
'Underworld' (1997)
Underworld is so much longer than any other book by Don DeLillo, because other than this one, his novels rarely exceed more than 300 to 400 pages. Page count alone doesn’t necessarily make something an epic (though it definitely helps), with the span of time covered in Underworld making it more than worthy of inclusion here, rather than anything else.
The book aims to paint a portrait of America throughout the second half of the 20th century, with a historical event kick-starting the narrative (with that narrative concerning the ball hit for the Shot Heard 'Round the World in 1951). From there, Underworld travels forward in time to the 1990s, and then keeps going back, decade by decade, until things arrive in the 1950s once more. There are lots of characters and tangents along the way. It’s almost overwhelming, at least at times, and certainly epic in feel, by the end.
'The Dark Tower VII: The Dark Tower' (2004)
Unlike Don DeLillo, Stephen King has written numerous books that are incredibly long. Thankfully, he’s good at writing epic-length stories, for the most part (there is The Tommyknockers, admittedly, but that one’s easy to eat around, like a single poorly-prepared vegetable on an otherwise tasty dish). The Dark Tower VII: The Dark Tower is one of his longest, and also up there among his best books overall.
The whole The Dark Tower saga is epic, admittedly, and also maybe the kind of thing you can count as one huge story, so consider those other books in the main series honorably mentioned. But The Dark Tower VII is particularly long, and it reaches some immense emotional highs while bringing the whole journey to the titular Dark Tower to an end that’s somewhat inevitable, and then also quite surprising. Big swings are taken here, and, truth be told, most of them hit. Or are hits. Whatever the correct baseball metaphor is (did not manage to hear that shot from Underworld… sorry).
'His Dark Materials' (1995–2000)
Since you can obtain all three entries in the His Dark Materials trilogy in one book, it’s being counted as its own story here. It’s the kind of thing where it’s hard to imagine someone reading Northern Lights (or The Golden Compass, if you're ‘Murican) and then not proceeding onto The Subtle Knife, with that second entry in the trilogy possibly being even better than the first.
The third, The Amber Spyglass, is a bit wild, but still essential to the narrative. Overall, His Dark Materials does a lot as a young adult fantasy trilogy that has more than enough to offer for actual/older adults, and there’s quite a bit by way of science and philosophy alongside the fantastical elements, too. If you approach it as one thing, then it definitely feels like a more-than-substantial epic narrative.
'Swan Song' (1987)
Swan Song concerns the end of the world, and a conflict over what remains that continues being fought, even when so much has ceased to exist, which makes it comparable to a certain Stephen King book, but more on that one later. Here, the devastation happens because of a short-lived nuclear war that decimates the planet, with certain supernatural occurrences happening after the world’s end, too.
It’s huge, but never overwhelmingly so, and it feels like one of the best horror/post-apocalyptic books of all time without any kind of TV or movie adaptation yet. That makes Swan Song feel kind of underrated and overlooked, and any imperfections prove quite easy to overlook when there’s so much to admire and get swept up in (something that can be said about many great epic-length stories, truth be told).
'Infinite Jest' (1996)
In Infinite Jest, there’s a tennis academy, with a bunch of kinda quirky students living there, playing tennis (surprise!), and then doing some other stuff. There’s also a drug and alcohol recovery house, and characters there grappling with addiction. Then, there’s a thread of Infinite Jest that plays out like a family drama, which is sort of connected to the tennis academy. And then you’ve got the radical freedom fighters known as the Wheelchair Assassins.
There are some other things that drive the narrative, or the narratives, because Infinite Jest is beyond sprawling, and not all that concerned with making you see how everything connects.
All these groups are living in a strange future that might not be all that strange, or different from the actual future, as David Foster Wallace imagined it back in 1996. There are some other things that drive the narrative, or the narratives, because Infinite Jest is beyond sprawling, and not all that concerned with making you see how everything connects. Oh, and there are endnotes. So many endnotes. You can spend a few dozen hours getting through Infinite Jest before realizing you'll have to spend a few dozen more re-reading it, or even studying it. But it’s worth it, even if you only understand or connect with a fraction of the overall text.
'Mason & Dixon' (1997)
There are other Thomas Pynchon novels that are potentially definable as epics, including the gonzo World War II/sci-fi/borderline-horror/postmodern novel that is Gravity’s Rainbow, and the slightly less confronting (albeit even longer) Against the Day. Mason & Dixon, though, might well feel the most like a traditional epic, with the characters being two real-life people, but their exploits in the novel itself straying from reality.
Charles Mason and Jeremiah Dixon are the protagonists, and Mason & Dixon is sort of about them establishing the Mason-Dixon line, but they're also part of a story within a story, and that storyteller does embellish a great many things. Also, Mason & Dixon is a uniquely written epic of sorts, because the style is meant to mirror/parody literature from the 1700s (the century during which the novel is set).
'The Stand' (1978/1990)
Another Stephen King book, yes, but The Stand is worth including here. It was originally published in 1978, and then there was an uncut version published in 1990, which is even longer, and then it was reworked so that it took place in 1990, rather than 1980. Either way, you’ve got a narrative that concerns the end of the world, like Swan Song, but unlike Swan Song and its nuclear war, the end of the world here comes about because of a deadly virus.
Things get epic in The Stand after the end, so to speak, with survivors joining one of two factions, and then those factions battle over the future of the world (or what’s left of it). There are so many characters here, and a great deal that happens across either about 800 pages, or approximately 1200, depending on which version of The Stand you read.
'The Winds of War' (1971) and 'War and Remembrance' (1978)
This is a duology, admittedly, but The Winds of War and War and Remembrance were conceived as the one epic, and both are about World War II. Herman Wouk wrote extensively about the topic, though, and included a great many characters both fictional and historical, so The Winds of War ended up being just one half of the story, concluding with the attack on Pearl Harbor.
The focus here is on World War II as it related to the United States, and so War and Remembrance, picking up after Pearl Harbor, does feel like a logical part of the overall story. Taking both parts together, it might well be the definitive English-language epic about the Second World War, with it also succeeding as one of the best works of historical fiction of the entire 20th century.
'War and Peace' (1869)
More war, but one that happened much earlier than World War II, War and Peace is about Russia and the Napoleonic Wars, broadly. It’s risky being broad when there are something like 1400 pages here, and the look at how this conflict shaped Russia and affected countless people is extensive and very detailed, with the detail making up for the narrative “only” spanning about a decade, for the most part.
War and Peace showcases that a lot can happen in a decade, though, to put it mildly. It’s hard to call it Leo Tolstoy’s definitive masterpiece when Anna Karenina also exists, yet War and Peace is longer and feels like more of a full-on epic (perhaps even being too overwhelming with information and general stuff happening for some), so it’s the one that feels most worthy of being included here.
'The Lord of the Rings' (1954–1955)
Does The Lord of the Rings need to be introduced, at this point? Further, can it be introduced/summarized? Like, the main narrative here is about destroying a Ring that was first properly introduced in The Hobbit, but there’s so much more happening beyond that, with a massive war being fought and so much being at stake, with things feeling all the more monumental the more you get invested in the world J.R.R. Tolkien created.
You get a good feel for said world within The Lord of the Rings, but the core narrative… okay, it does a bit more than scratch the surface, but there is also so much beneath that surface. The accomplishment here, as an expansive work of literature, is quite staggering in ways people have been trying to wrap their heads around for decades upon decades now, so it feels hard to justify putting any other work other than The Lord of the Rings in the #1 spot here, in all honesty.