I am defending a battlefield with Fiora, Peerless. My opponent attacks it with a 1 unit. If I react with an ambush unit, does my Fiora's might get doubled? What if instead I use Disciple on my Fiora, is it at 8 might or 10 might? Also, what's its interaction with Master Yi, Wuju Bladesman?

When defending with Fiora, Peerless, her might-doubling ability is a triggered ability that checks its condition—being "one-on-one"—only at the moment she is designated as the defender and the trigger is placed on the initial chain.

Here is the breakdown of your questions:

1. Ambush and Fiora's Might

If you react to the attack trigger by playing a unit with Ambush to your battlefield, Fiora's Might will still double.

• Reasoning: As established in RiftJudge FAQs #1231 and #577, the "one-on-one" condition is checked at the moment of designation. Because the battlefield was one-on-one at that moment, the trigger is placed on the chain. Adding more units to the battlefield later via Ambush does not retroactively cancel or prevent the trigger from resolving.

2. Interaction with Discipline

If you play Discipline (assuming this is the spell you meant) on your Fiora, she will have 10 Might, not 8.

• The Sequence:
  1. Initial Chain: The showdown begins, Fiora is designated, and her "When I defend" trigger goes on the chain.
  2. Closed State: The game enters a Closed State. Because Discipline is a [Reaction], you can play it while the chain is closed.
  3. Discipline Resolution: Discipline resolves first, giving Fiora +2 Might. Her current Might increases from 3 (base) to 5.
  4. Trigger Resolution: Fiora’s trigger then resolves, doubling her current Might (5).
  5. Final Result: 5 × 2 = 10 Might.
• Note: You must ensure you play Discipline in response to her trigger. If you wait until the chain is empty, the trigger will have already resolved on her base Might (3 × 2 = 6), and adding the +2 later would only bring her to 8.

3. Interaction with Master Yi, Wuju Bladesman

When Wuju Bladesman is active, it provides a passive ability (+2 Might to a lone defender). This interacts with Fiora as follows (consistent with RiftJudge FAQ #10960):

1. Passive Application: As soon as Fiora defends alone, the +2 from Wuju Bladesman is applied in the Arithmetic layer. Fiora's Might becomes 5 (3 base + 2 passive).
2. Triggered Ability: Fiora's "When I defend" trigger is placed on the chain.
3. Resolution: When the trigger resolves, it doubles her current Might (5), resulting in 10 Might.

If you use both Discipline and Wuju Bladesman in the same combat, the passive +2 applies first, Discipline adds another +2 (total 7), and then the trigger doubles that to 14 Might.