Is a game's RTJ thread limited to 50 posts?
No game threads have a limit on their length, and that would include RtJ threads. The nearest would be that when you hit 1000 posts, you can no longer view the whole thread.
And checking the game, there are other RtJ threads there that have almost 100 posts, so definitely not any sort of site limit.
I'd recommend starting a new thread to continue the discussion if it's urgent, since the only person who can fix it (if its some sort of glitch) might not get to it for quite some time.