How many in the last couple of years. Vesey, Butcher, Wheeler, Kerfoot. That's just out of my head without doing any research. Pretty good players too.
There is a lot of mix up as to which is the loophole in the media and with fans. The loophole is for any players that don't go directly to the NCAA. If you draft a kid out of the USHL, USHS, etc.. and the next year they don't go to the NCAA, then after their 3rd year they can inform the team that drafted them that they will not sign with them and instead are opting to become a UFA on July 1st. If said player goes directly to the NCAA they can not do this, but what they can do is after their Senior year they can refuse to sign and become a UFA on August 15th.
Wheeler is the only one you named that actually used the loophole. Mike Reilly is another, drafted by jackets, after his Junior year he signed with the Wild. Justin Schultz is another drafted by the Ducks, left after his Junior year to sign with the Oilers.
Vesey and Butcher weren't even eligible for the loophole. Kerfoot was. For the Habs, Jake Evans wasn't eligible, neither is Primeau, Koberstein, or Poehling. Hawkey is eligible at the end of this season.
Now many people call it a loophole because after their Senior year if said player refuses to sign they can become a UFA but to me that's not really a loophole. CHL players can opt not to sign as well just like Euro players, the difference with CHL players is age, if they do it they might still be young enough to re-enter the draft.
Here's an article on it from THN, they call it a loophole as well but I don't agree with it but still a decent read on the subject and how it's not used much.
NHL doesn't have an NCAA 'loophole' problem, it's just business as usual | The Hockey News