Elvish Warmaster

It doesn't matter how many Elves enter the battlefield under your control. The ability creates only one Elf Warrior creature token.

Once the triggered ability has triggered once during a turn, it can't trigger again, even if the triggered ability is still on the stack, has been countered, or has otherwise left the stack.

The activated ability affects only Elves you control as the ability resolves. Elves you begin to control later in the turn won't get the bonuses. Creatures you control that become Elves later in the turn won't get the bonuses, and Elves you control that got the bonuses that stop being Elves later in the turn won't lose them.