Guild Wars 2

For Empowered, if I have 25 stacks of might, do I get the increase in damage once or 25 times?

Might is 1 boon, empowered = for every boon. 25 * 1.3 = 32.5% increase to damage.

That would be seriously overpowered, and if Empowered is working like that something is wrong, I can tell you without even consulting A-Net, because Empowered is not intended to work like that.

Just like removing one condition removes all stacks of the condition removed, applying damage based on boons applies once regardless of stack number.