have a Mask of Foresight on the board and I have a unit defending Zaun Warrens. My enemy attacks Zaun Warrens, kills the unit, and conquers. In response to the Zaun Warrens trigger, I play Rengar Trophy Hunter. When combat starts again, how much might does Rengar have?

Rengar has 7 Might — his base 6 plus 1 from Mask of Foresight.

Mask of Foresight triggers whenever a friendly unit gains the attacker/defender designation while alone ("When a friendly unit attacks or defends alone, give it +1 might this turn"). When the new combat opens, Rengar is your only unit there, so he's the lone designation and the Mask fires for him (you don't get penalized because the earlier defender already took a Mask buff — it's per-trigger, and that unit is dead). The +1 resolves on the combat showdown, so it's live for damage.

One wrinkle worth noting: because Zaun Warrens was already conquered, this isn't the usual "surprise defender" Rengar line — the battlefield was no longer contested when he arrived, so Rengar re-applies Contested and enters as the Attacker, not the Defender (FAQ #7053). Either way the Mask triggers (attacking alone counts too), so the answer stays 7.