Nimble Trapfinder

Nimble Trapfinder’s first ability checks only whether a permanent entered the battlefield under your control earlier in the turn and had one of the party creature types as it entered. It doesn’t apply if an entering object gained the type later in the turn. It does apply if the entering object left your control since. It doesn’t matter whether Nimble Trapfinder was under your control as that object entered.

An ability referring to the number of creatures in your party gets a number from zero to four. Such abilities never ask which creatures are in your party, and you never have to designate specific creatures as being in your party. You can’t choose to exclude creatures from this count to lower the number.

After Nimble Trapfinder’s second ability has resolved, creatures you control keep the triggered ability for the turn even if you no longer have a full party later in the turn.