Does Kha'Zix legend card where I spend XP to buff 1 unit allow me to buff multiple different units in 1 turn?
Yes, you can use the ability on Voidreaver to buff multiple different units in one turn, provided you have enough XP to pay the cost for each activation.
According to the card text for Voidreaver, the ability is:
"Spend 1 XP : [Buff] a unit."
Can you use it multiple times?
Yes. As established in RiftJudge FAQ #6205, the "Spend XP: effect" pattern is an activatable ability that can be activated as many times per turn as you can pay the cost.
In summary:
- There is no limit to how many units you can buff in a turn, provided you have the XP to pay the cost for each instance.
- Each activation is independent and requires the payment of 1 XP.
Note: I could not fully verify this ruling against the retrieved rules, as the provided text did not explicitly confirm the presence or absence of an exhaust cost.