Pulling from [this unofficial tweet from November 2018](https://twitter.com/JeremyECrawford/status/935230061967392769) 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.