If a creature you control would enter the battlefield with a number of +1/+1 counters on it, it enters with that many plus one instead.

Conclave Mentor's first ability doesn't apply to itself if it's somehow entering the battlefield with a +1/+1 counter on it.

If you control two Conclave Mentors, the number of +1/+1 counters put on a creature is two plus the original number. Three Conclave Mentors add three, and so on.

If two or more effects attempt to modify how many counters would be put onto a creature you control, you choose the order to apply those effects, no matter who controls the sources of those effects.

Use Conclave Mentor's power as it last existed on the battlefield to determine how much life you gain.