Can I react to Fiora Peerless when I attack or defend trigger with Ambush so it doesn't gain Might for one on one?

No, reacting with a unit that has Ambush will not prevent Fiora, Peerless from gaining the Might bonus.

According to RiftJudge FAQ #11168 and #1231, Fiora, Peerless's "When I attack or defend" ability checks the "one-on-one" condition only at the exact moment she is designated as the attacker or defender.

The Sequence

1. Designation: Fiora is designated as the attacker or defender. If the battlefield is one-on-one at this exact moment, her ability triggers and is placed on the initial chain.
2. Trigger Evaluation: Because the condition was met at the moment of designation, the ability is now on the chain.
3. Reaction: The game state is now Closed, meaning you may play a unit with Ambush as a [Reaction]. However, playing an Ambush unit now (or any other effect that changes the battlefield state) happens after the trigger has already been successfully added to the chain.
4. Resolution: Because the ability uses "when" (a triggered ability) rather than "while" (a passive ability), it does not retroactively check the condition when it resolves. It will resolve and double Fiora's Might regardless of whether more units were added to the battlefield after the trigger was placed.

This is a deliberate mechanic of Fiora, Peerless—once her trigger condition is met and it is placed on the chain, her Might bonus for that combat is guaranteed.