A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesnâ€™t matter how long the permanent has been a creature.
The value of X is determined when the ability resolves. It wonâ€™t change later, even if the number of Elves you control changes.
If you control no Elves when the ability resolves, the Forest will become a 0/0 creature and be put into the graveyard. However, since Elvish Branchbender is itself an Elf, the Forest will usually be at least a 1/1.
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