OK, I'll pile on. Any good guitar shop, and many general music stores will sell singles, usually by actual size, so 'good to know what you need. When you 'pays your money, you get a "packet" ...wait for it... with one string.
Back to whatever. If you're going for your first new one(s), you may even want to take the guitar along for advice on strings to buy. Skipping 200 words, I'd recommend buying two complete sets, plus a couple of extra singles for at least the 1st & 2nd strings, the ones most often broken (and those little ones are really cheap).
With a set, the size of each string that makes the set is right on the front of the package. So for example if I decide on a light electric set with a .010 first string, I'll get a couple of single .010's too.
What you keep on hand kinda depends on how hard and often you play.
But you should at least have a full new set in the case, always. If you just use one from it - keep the "packet" it was in, and go buy a 'single.