I don't see any limitation (other than the # of sorcery points). I think each Hound can attack a different creature (or all the same), but each can only attack one creature designated as a target. Good if you need cannon fodder and don't have any other caster (or one of the enemies can cast divine word, which has a good chance of taking out all summoned enemies, since monstrosities aren't subject to its banishing effect), but 3 SP's can eat into your budget in a hurry. I am interpreting the language as you can summon 1 per round (specifically "At 6th level, as a bonus action you can spend 3 Sorcery Points to magically summon a hound of ill omen" [bolding mine]) , using a bonus action, so a pack might take a bit.
In terms of per creature capability, it is competing with conjure animals, conjure minor elementals, conjure woodland beings, and summon minor demons, so if you think upcasting any of those spells is a good idea, then spamming this is a good plan.