The advice would be not to attend until you are reasonably confident of passing rather than ponder how many fails you're planning before passing.
If you fail 2 POCs, the odds are Officer is not for you anyway. If having failed two POCs you then fail a PRMC, the odds are the Royal Marines isn't really going to be your "cup of tea" in any event.
Occasionally we get the odd individual who seems to think that if they attend CTC a sufficient number of times, they're bound to pass, but sadly it never works out that way.