Some people think you can live with one pair of sheets. Those people have NEVER ACHIEVED ANYTHING.
Oh, sure, you can have a pair of favorite sheets, absolutely you can. And you can totally use them as much as you can. But what about that time at 4 in the morning when the dog gets sick? Won't you need extra sheets then? Or what about if you have kids, and they're visiting your bed, and there's a little accident. Do you want to wait to wash your single set? Or hey, what about this: pretend you've got someone who wants to stay over on two consecutive nights. How lucky for you! And do you REALLY want them finding out you've been using the same sheets night after night after night?
The sheet flow chart is easy. If you don't have sheets, you need sheets. If you do have sheets, you need extra sheets. If you have extra sheets, you need backup extra sheets.