Depends. I would change FWD car tyres in pairs. Keep the good one and make a pair out of it and another tyre.
Some 4WD cars need all tyres changing at once as a significant wear difference can speed up wear in the centre diff. When I had a Quattro, I rotated the tyres front to back regularly to keep the wear even. Still never got more than 15k miles out of a set of 255/35R19Y tyres.