That of the limited number of slots for the custom classes was one of the problems the players of Call of Duty Modern Warfare they complained, above all because the Prestigio system no longer exists and therefore, unlike the previous titles, it is not possible to unlock new ones.
Fortunately though Activision plans to change things, and to make more slots available soon, as confirmed by the art director of Infinity Ward Joel Emslie, who replied to some feedback on Reddit.
"You and everyone who has commented on it, you are right. It is one of the main problems discussed in the studio, and we are working on it. I am not sure which update will arrive, but I believe in the next one. Believe it or not, it can cause tremendous bugs if not tested properly. I'm not entirely sure though, so don't blame me if I'm wrong. But since I'm commenting here, I'll put pressure on the studio to fix it. "
In short, it will take a little patience, but if the issue is resolved already in the next update, we are sure that Activision will make many players happy. In the meantime, on our website you can read all the latest news of the Call of Duty League and, of course, our review of Call of Duty Modern Warfare.