Unit is one might. I stupify it. Then they buff it +2 with action speed. How much is the unit now?

3 might.

Stupefy's −1 can't take the unit below 1, so on a 1-Might unit it does nothing — the unit stays at 1 (Rule 477.3.b snapshotting). The +2 buff then brings it to 3.

Order matters if the buff was played in response on the Chain: the +2 would resolve first (unit → 3), and then Stupefy's −1 applies to the current 3, leaving it at 2. Since you described Stupefy resolving first, the unit ends at 3.