I just went through this recently. Two failed PPEs before I found one that passed. There is no one-size-fits-all answer to the "how many times" question. You could vet 10 horses and have them all fail, or have the first one pass the first time. I think your best bet would be to make an educated evaluation before having the PPE done -- ask lots of questions about the horse, its history, watch it go, look it over physically with an educated trainer or professional, etc., and also consider how much horse you're getting for the price. If you're on a limited budget, which it sounds like you might be, keep in mind that if it sounds too good to be true, it probably is -- like if you're getting a proven 3'6" horse for $10,000, there's probably something wrong. Sometimes you can do all that and they still won't pass, in which case you can either reconsider your priorities (is whatever issue REALLY that much of a dealbreaker?) or thank your lucky stars you paid for the PPE and not for a lame horse.