As the new M2 has 50% more HP up to 18750, can we still trio it or it will need at least 4 players? Simple calculation shows that 18750/300/3 = 20.83 and 18750/300/4 = 15.625. It seems under proper weather and with friendship boosts, trio is still possible theoretically, but the unavoidable re-lobbying may kill the chance of success. Yet I believe Quartet is pretty safe with adequate counters.
Simple math gives us the answer, before lobbying issues. For your personal results, time your reviving.
If you can have duoed it before, you can trio it now.
If you could just barely have trioed, you'll need 5 minimum (4.5, rounded).
If you could have managed 37.5% damage before, you can quad it. (A trio of that power would do 112.5% of original Mewtwo health, plus the need for an additional 37.5% to make up the difference.)
Trio will require an optimal counter, good internet connection, favorable Mewtwo raid boss moveset, and luck
4 man it still give you a good chance and more forgiving in counter selection, unless if its Confusion- Focusball Mewtwo and your squad is heavily based on Gengar and Tyranitar.
Manage to win with 4 trainer, no planning beforehand, just try it on the spot with my fellow trainer. One trainer still use 2 Houndoom, one trainer making mistake of using smackdown TTar when rejoin. Win against Confusion-Thunderbolt with 7 second left