If Yi summons Rengar to the battlefield with Ambush when there is 1 unit at the battlefield, will the +2 might from Yi fall off or will the unit retain +2?

The unit will lose the +2 Might bonus immediately upon Rengar, Trophy Hunter entering the battlefield.

According to RiftJudge FAQ #10141 and #3935, Master Yi's ability (from Wuju Bladesman, Starter) is a continuous passive effect with a "While" condition. Continuous effects are checked constantly and update the game state instantaneously. The moment a second friendly unit enters the battlefield, your original unit is no longer "alone" (as defined by Rule 741.1), and the bonus is removed.

Timing Sequence:

1. Opening State: A unit is defending alone. Master Yi’s continuous effect applies, granting it +2 Might.
2. Ambush [Reaction]: You play Rengar, Trophy Hunter using the Ambush keyword. This is a [Reaction] that is added to the chain.
3. Resolution: The chain resolves, and Rengar enters the battlefield.
4. State Update: The instant Rengar enters the battlefield, the "While" condition for Master Yi’s passive is no longer met. The +2 Might bonus is removed immediately, regardless of what other items may still be on the chain or the fact that this occurred during a reaction.

This interaction is distinct from triggered abilities (which check conditions only at the moment they trigger and are placed on the chain); because Master Yi’s ability is a continuous passive effect, it does not "snapshot" and instead recalculates as soon as the board state changes.