If I conquer with a Rengar pouncing and Rumble hot headed, will Rengar's might be 5 or 3 when I recycle Rengar to use Rumble's ability?

When you conquer with Rengar, Pouncing and Rumble, Hotheaded, Rengar’s Might will be 5 when you recycle him for Rumble’s ability.

Reasoning

According to the RiftJudge FAQ (ID 8712), when you recycle a unit as a cost for an ability like Rumble, Hotheaded, the ability "looks back" at the unit's characteristics immediately before it left the board.

1. Combat Cleanup: As established in RiftJudge FAQ (ID 9610) and FAQ (ID 6972), during the combat cleanup that occurs at the end of a showdown, units at the battlefield gain the "Attacker" designation.
2. Assault Activation: Once Rengar has the "Attacker" designation, his Assault 2 keyword activates, increasing his Might from 3 to 5.
3. Cost "Look Back": Because the recycle action for Rumble’s ability happens as a cost while the unit still has its board-state characteristics, the ability uses the Might value Rengar had on the board at that moment (5), not his base Might (3).

Note: This interpretation is based on the "look back" rule for costs established in RiftJudge FAQ #8712, applied to the timing of combat cleanup and the Attacker designation for Rengar.