You choose up to one target creature total, not a separate target for each Zombie that player controls.
2016-07-13
If the targeted creature becomes an illegal target, the targeted player will still get Zombie tokens.
2016-07-13
You may cast Dark Salvation with X as 0 just to have a creature get -1/-1 for each Zombie a player controls, and you may cast it without targeting any creature just to have a player get Zombie tokens. If you really just want to cast a spell, you can even cast it with X as 0 without a target creature and have neither effect do anything.
Target player creates X 2/2 black Zombie creature tokens, then up to one target creature gets -1/-1 until end of turn for each Zombie that player controls.
