I usually buy two at a time. The driving wheels wear out tires faster. I don’t rotate tires; I figure they rotate plenty while I’m driving. In my broke younger days, I’d buy one used tire whenever I absolutely had to.
I used to buy used tires. Until coming back from a trip I got two flats within the span of a couple of hours. I changed the one tire, but then had to get a tow in the middle of the night for the second one. And a motel room.
I figured new tires would be cheaper!