One. It is a problem.
You are skeptical; I can tell. You say: "What? One is literally the simplest thing in existence. How can it be a problem?" Well, dear reader:—that is precisely the issue.
Consider, by contrast, two. If I have two mugs, say, then they are two. And I can tell that they are two because this one is green and this one is blue, for example. They are distinct.
—Okay, but what if they are the same colour?—No problem. I can take a piece of chalk—the sage's timeless tool—and draw a line on my floor and put a mug on one side and a mug on the other, and there are both mugs. We've divided mug from mug, we've divided them in twain, and we've got two.
We can keep going, too—more distinctions, more numbers. Doesn't have to be mugs. Could be pancakes. Could be piles of salt. But it's better not to think about piles of salt — that one has its own problems. Point is, now that you have two, you can draw two lines of chalk, and then you can distinguish between three mugs. So now you have three. You can go on that way as long as you like—or, practically speaking, until you run out of chalk. I doubt anyone's broken through the tens of thousands. Who wants to hold a piece of chalk for that long?
So much for multiplicity. But forget the chalk, and—say we have got 1000 mugs—scrap 999 of them. Then: what have we got? We have got mug, for sure: but one mug? There is no number here, no counting is necessary; one need not enumerate, but only point and say, "That!". How can we say one apart from the potential for two? Where does one belong, except at the beginning of the sequence one, two, three, …? It seems, indeed, that one is only one among multiplicity; if there were truly only one—that is, if multiplicity were excluded from the essence of mug, then one could not properly call it one either.
Some religions, for example, have a god which they proclaim to be utterly one: this can only be a figure of speech that contrasts their god with the case of other religions which do have multiple gods (even though the god of the one religion and the gods of the other are unrelated ontologically, but share only a noun); in one peculiar case, the god in question is said also to mysteriously be three, and its oneness becomes comprehensible in contradistinction to its threeness — a rather creative solution to this timeless problem.