What? That's not how holy works. Imagine tank pulls a crowd of 10 mobs and you holy once.

1st mob takes full potency (200 potency)
2nd mob takes -10% damage compared to the 1st (180 potency)
3rd mob takes -20% damage compared to the 1st (160 potency)
4th mob takes -30% dmg compared to the 1st (140 potency)
5th mob takes -40% dmg compared to the 1st (120 potency)
6th, 7th, 8th, 9th and 10th mobs take -50% damage compared to the 1st (100 potency)

So if you spam holy on single target, that same mob will take the 200 potency consistently, if you spam holy on 2 targets, one will take 200 potency and the other will take 180 potency consistently and so on