Does Ahri Legend apply -1 might to all attacking enemy units in my battlefield? Or just one. And does Deflect trigger on the enemy units?

Ahri Legend's ability applies to all attacking enemy units at a battlefield you control. It does not target, so it does not trigger Deflect.

Here is the breakdown based on the official FAQs:

• All units affected: The ability triggers individually for every enemy unit that gains the "attacker" designation at a battlefield you control. If multiple units attack together, the ability triggers once for each of them, and each receives the -1 Might reduction.
• No targeting: Because the ability automatically applies to any unit that attacks and does not require you (the controller) to choose a specific unit, it is not considered to "target" anything.
• No Deflect: Since the effect does not target, it does not interact with the Deflect keyword. Your opponent does not need to pay any Deflect costs for this ability.

This ruling specifically applies to the Ahri Legend card. Note that there is another version of the card, Ahri, Inquisitive, which does target and would trigger Deflect.