Apparently Cameron Lynn, Colin Cassady and Leroy Parks didn't get released after all, but the others definitely did.
If you go through the past NXT Rookies, it's tough to think of many that can be released:
NXT1 - Michael Tarver has already been released, Darren Young COULD be in trouble after NXT5, but the rest are all firmly on the main roster. Well, not so much Skip Sheffield, but if Vince McMahon is high on him then he isn't in any trouble.
NXT2 - Kaval and Lucky Cannon have already been released, McGillicutty and Riley are firmly on the main roster, and O'Neil is the favourite to win NXT5 and move on to NXT6. Husky Harris and Percy Watson both shouldn't be released, especially Harris as he's so unique. Eli Cottonwood is apparently injured at the moment, but surely he will be released as he is shit.
NXT3 - Who cares.
NXT4 - Jacob Novak has thankfully already been released, Brodus Clay is firmly on the main roster, and Johnny Curtis is about to debut for Smackdown this week. Bateman is on NXT5 and surely wouldn't be in any trouble if he doesn't win, but the same can't be said for Byron Saxton and Conor O'Brian. Both of them could follow in Lucky Cannon and Jacob Novak's footsteps as they both failed on NXT5 as well. Personally I find Saxton to be entertaining and I would rather him over O'Brian, but it doesn't look too promising for either.
In my eyes, only Eli Cottonwood should be released. If I had no choice and was forced to pick more, then I guess I'd have to go with Conor O'Brian and Percy Watson.