think your allowed to do it as many times as you want (not 100% though) within reason. most afco's wont book you on a prmc until you pass. i know someone whos been on 2 already and has his 3rd booked - but he did pass his first and is only doing it because he has delayed his prmc and the afco's letting him. but if he gets to go on them even though he's passed i dont see any reason why you shouldnt be allowed to do more than one if you fail. you'll probably have to wait a few months mind.