I reccon 6 - 8 It also depends on how often you want to wash too. I am an every 2nd day washer so that many covers suited my habits because at any one time there would be a couple in the bucket and a couple drying as well as the ones either on bub or ready to put on. There is nothing worse than stressing over not having enough stuff dry or resorting to using the dryer all the time.

You can get away with rotating them if they are just wet and don't get poos on them because they will dry out but you need some backups JIC you have poosplosions.