If I attack a battlefield with 2 Gemhand Hunters while Ahri holds, does she reduce the might of one of my units or both?

Ahri's ability triggers individually for each enemy unit that attacks.

If you attack a battlefield where Ahri is holding with 2 Gemhand Hunters, her ability will trigger twice—once for each of your units—and both Gemhand Hunters will receive the -1 Might reduction.

According to official FAQs (e.g., FAQ #5058, FAQ #2129), this ability does not target and does not just affect one unit; it triggers separately for every individual enemy unit that gains the "attacker" designation at that battlefield.