It is 50 contracts but I don't think that has anything to do with them choosing to keep both Slater and Copp as options on the 4th line. You still have a maximum number of players you can have up with the club.
The 50 contracts is the maximum any team can have including players signed that are in the minors and juniors I believe. This is actually one of the benefits of drafting players out of the NCAA and USHL, because you can't sign those players to a contract while they are down there (well NCAA for sure). So you don't burn one of your 50 on those players but still retain their rights. Being close to the max is more a result of how many prospects we have coming up.
I'd be surprised if Slater is back. With all the prospects now coming up the Jets really need to start adding at least 2 a year. Copp, Ehlers, Morrissey, Petan, Armia, all possibilities this year.
My money is on Copp and Ehlers.