If Fiora Peerless is defending alone with Master Yi Legend, and my opponent attacks with 1 enemy, do I go to 10 might or 8 might?

You go to 10 Might.

Here's why:

1. Master Yi's bonus (+2) is a passive ability — it applies immediately in the Arithmetic layer the moment Fiora is designated as the lone defender. Her Might becomes 3 (base) + 2 = 5.

2. Fiora's "When I defend" is a triggered ability — it goes on the Initial Chain. When it resolves, it doubles her current Might (which is already 5 thanks to Yi), so 5 × 2 = 10.

This is confirmed by FAQs #10960 and #11088 — the passive applies first, then the trigger doubles the boosted value. You don't double the base 3 to 6 and then add Yi's +2 (which would give 8). The layering order and the fact that passives are applied before triggers resolve guarantees 10.