Pests of Honor

Creature — Mouse

Celebration — At the beginning of combat on your turn, if two or more nonland permanents entered the battlefield under your control this turn, put a +1/+1 counter on this creature.

Each celebration ability has a different effect.
Some celebration abilities trigger at specific parts of the turn and check whether two or more nonland permanents entered the battlefield under your control already in that turn.
Others are static abilities that give creatures abilities or power and toughness increases as long as two or more nonland permanents entered the battlefield under your control that turn.
Celebration abilities only care if two or more nonland permanents entered the battlefield under your control in a turn. They won't get more powerful if more than two permanents entered the battlefield under your control in a turn.
The permanents that entered the battlefield don't need to remain on the battlefield or under your control. Celebration abilities are checking for past events, not the current game state.