ahri nine tailed fox triggers in one or all of the attackers

Each attacking unit triggers it separately — so effectively all attackers get -1, but it's one trigger per unit, not one trigger total.

Nine-Tailed Fox reads "When an enemy unit attacks a battlefield you control, give it -1 might this turn, to a minimum of 1 might." Because the condition is the singular "an enemy unit," the ability triggers once for every individual unit designated as an attacker (Rule 383.1), and each trigger goes on the chain separately, resolving for that unit. The Ahri player never chooses or targets a unit, so nothing is stopped by Deflect or untargetability — but note a trigger can be missed if you don't announce it before its observable impact (Tournament Rules 506.3).

One nuance: if a new unit joins combat later (e.g., played in via Ambush or moved with a spell), that's a new attacker and triggers Nine-Tailed Fox again on a fresh chain.