In order to activate the ability, you must be able to target a creature with power less than or equal to the number of creatures you control. The comparison is made again when the ability resolves. If the power is greater than the number of creatures you control at that time, the target is illegal and the ability is countered.

The control-changing effect lasts indefinitely, even if the power of the creature becomes greater than the number of creatures you control after the ability resolves.