Every single time a card refers to it's own name as an In Play card (and not some other card with the same name in the discard, the deck, or your hand) it is referring to itself and itself alone.
You can use it once for each Unown X that is on your bench. Each one uses it's own power and it can use that power once, as long as it is on the bench.
Each of them would have to be on the bench.
If you have one active, that one could not use it's Power unless and until it became Benched.
And if you somehow move it around from bench to active and back to bench, you still only get to use that specific X's power once. It hasn't left play, so it hasn't forgotten that it used it's own power.
I'm not sure what significance you place on the word "have" in this case. It doesn't chance the meaning.
