Conclave Mentor

• WWizards of the Coast2020-06-23
  Official Ruling
  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.
• WWizards of the Coast2020-06-23
  Official Ruling
  Conclave Mentor's first ability doesn't apply to itself if it's somehow entering the battlefield with a +1/+1 counter on it.
• WWizards of the Coast2020-06-23
  Official Ruling
  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.
• WWizards of the Coast2020-06-23
  Official Ruling
  Use Conclave Mentor's power as it last existed on the battlefield to determine how much life you gain.
• WWizards of the Coast2020-06-23
  Official Ruling
  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.

Rulings via Scryfall