Blood Researcher

• WOTC Official Reminder Textofficial rules
  Menace — This creature can't be blocked except by two or more creatures.

• WWizards of the Coast2021-04-16
  Official Ruling
  If you gain an amount of life “for each” of something, that life is gained as one event and the ability will trigger only once.
• WWizards of the Coast2021-04-16
  Official Ruling
  An ability that triggers “whenever you gain life” triggers just once for each life gain event, no matter how much life you gain.
• WWizards of the Coast2021-04-16
  Official Ruling
  If you gain life at the same time Blood Researcher is dealt lethal damage, it dies before its ability can put a +1/+1 counter on it.
• WWizards of the Coast2021-04-16
  Official Ruling
  Each creature with lifelink dealing combat damage causes a separate life gain event. For example, if two creatures you control with lifelink deal combat damage at the same time, a “whenever you gain life” ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.

Rulings via Scryfall