If an opponent plays Kha'zix Ambush to a battlefield where I am alone, and I react with Inferna Ambush so I have 2 units there, does Kha'zix have 4 or 6 might?

In this scenario, Kha'Zix, Mutating Horror will have 6 Might.

Reasoning

According to the established game rules and RiftJudge FAQs (e.g., #9849, #10198, #10465), the condition for Kha'Zix's triggered ability ("if an enemy unit is alone here") is evaluated at the exact moment the unit gains the "Attacker" or "Defender" combat designation upon entering the battlefield.

1. Trigger Condition Check: When you play Kha'Zix via [Ambush], he enters the battlefield and gains his combat designation. At that specific moment, the game evaluates the board state: "Is an enemy unit alone here?"
2. Condition Met: Since you are currently alone at that battlefield, the condition is satisfied. Kha'Zix's triggered ability is placed on the chain.
3. Reaction: Your opponent (you, in this context) responds by playing Inferna using [Ambush]. This [Reaction] adds Inferna to the chain above Kha'Zix's trigger.
4. Resolution (LIFO):
   • Inferna finalizes and enters the battlefield.
   • Kha'Zix's triggered ability resolves. Because the condition was met at the time the ability was triggered and placed on the chain, the subsequent change to the battlefield state (adding Inferna) does not retroactively negate the trigger.
   • Kha'Zix receives the +2 Might bonus.

Since Kha'Zix, Mutating Horror has a base Might of 4, adding the +2 bonus from the ability results in a total of 6 Might.

Note: This is my interpretation based on standard Riftbound chain mechanics and existing RiftJudge FAQs for this specific interaction. No official rule explicitly details the exact timing of "alone" checks outside of the context established by the referenced FAQs.