If I attack with multiple units with Disarm and Ahri, Confident, does Ahri get +1 for each instance of disarm or does it count as a single -x?

Radiance rules aren't published yet. This uses the printed card and current rules only, and it isn't a ruling on the new mechanics.

Yes — Ahri, Confident gets +1 for each separate instance of reduction, so attacking with multiple Disarm units at her battlefield is +1 per reduction that actually happens, not one lumped bonus.

Each attacking unit's Disarm is its own Attack Trigger and each one goes on the chain and resolves separately (rule 831.1.b, 831.1.d), so each one is a distinct "you reduce the Might of an enemy unit here by 1 or more" event for Ahri's second ability. That includes her own Disarm when she attacks. The only catch is the wording "here" — the unit being reduced has to be at Ahri's battlefield, and the reduction has to really land; if it's snapshotted to 0 (already at a floor), nothing was reduced and she gains nothing (FAQ #12942).

By contrast, a single unit disarming for -2 (base Disarm plus a granted Disarm, e.g. Clanging Sentry) is one event and only gives her +1 — rule 831.1.d.1 collapses multiple Disarm instances on one unit into a single trigger with the summed value, so "by 1 or more" counts the event, not the points.

This is my interpretation based on rule 831 and the printed card. No official FAQ exists for this specific interaction.