I have a battlefield I have one unit on and a hiding card and my opponent moves to my battlefield he wants to play Kha'Zix, Mutating Horror from hand but I activate my hiding card (Pyke, Dockside Butcher) on the ability on ambush on reactive speed does Kha'Zix effect still go on?

Yes, Kha'Zix, Mutating Horror will still receive its bonus.

According to the rules for triggered abilities and established RiftJudge FAQs (e.g., #10198, #10748), the condition for Kha'Zix's ability ("if an enemy unit is alone here") is evaluated at the exact moment the unit gains the "Attacker" or "Defender" combat designation.

Here is the step-by-step sequence of how this interaction resolves:

1. Kha'Zix Enters: Your opponent moves Kha'Zix, Mutating Horror into your battlefield. He gains the "Attacker" designation.
2. Trigger Condition: At this exact moment, the game checks the battlefield state: "Is an enemy unit alone here?" Since you only have one unit, the condition is met. Kha'Zix's triggered ability is placed on the chain.
3. Reaction: You use the [Reaction] timing granted by the Hidden keyword to flip your facedown Pyke, Dockside Butcher and play him to the battlefield.
4. Chain State: Playing Pyke adds him to the chain above Kha'Zix's triggered ability.
5. Resolution (LIFO - Last-In, First-Out):
   • Pyke enters the battlefield first.
   • Kha'Zix's triggered ability resolves. Because the "alone" condition was already satisfied at the moment Kha'Zix entered and triggered, the subsequent addition of Pyke to the board does not retroactively negate the trigger.

Even though there are two units on your battlefield when the ability resolves, the game does not re-evaluate the condition; it only checks if the requirement was met at the moment of the trigger. Therefore, Kha'Zix receives the +2 Might bonus and 2 XP.