Pulling from this Sage Advice column, the intention of the writing is that you can cast the ability as many times as you like, but can only affect a single target at a time. The lack of wording limiting the casting of Slayer's Prey supports the idea that it is an at-will ability.

Pulling from this unofficial tweet from November 2018 by rules designer Jeremy Crawford, the intention of the writing is that you can use the ability as many times as you like, but can only affect a single target at a time. The lack of wording limiting the usage of Slayer's Prey supports the idea that it is an at-will ability.