Player B has a unit at battlefield, player A moves Fiora Peerless into the battlefield, and player B plays Kha'Zix with Ambush. Does Fiora still get double might?

Yes, Fiora, Peerless will still double her Might.

According to the established game rules and RiftJudge FAQs (e.g., #11178, #11168, #1231), the "one-on-one" condition for Fiora, Peerless is checked only at the exact moment she is designated as the attacker or defender.

The Timing Sequence

1. Designation: You move Fiora, Peerless to the battlefield, initiating combat. Because she is the only unit you control there and Player B has one unit, the battlefield is "one-on-one" at that exact moment.
2. Trigger: Her triggered ability ("When I attack... double my Might") is placed on the Initial Chain.
3. Reaction: The game state is now Closed. Player B plays Kha'Zix, Mutating Horror using his [Ambush] keyword as a [Reaction]. This adds Kha'Zix to the chain above Fiora's trigger.
4. Resolution (LIFO - Last-In, First-Out):
   • Kha'Zix finalizes and enters the battlefield.
   • Fiora's triggered ability resolves.

Because Fiora's ability is a triggered ability ("When..."), it does not re-check the condition upon resolution. Since the "one-on-one" condition was satisfied at the moment of designation, the Might bonus is locked in once the trigger is on the chain. Adding Kha'Zix to the battlefield afterwards does not retroactively negate the trigger.