Well, first of all, I love Star Trek, so your post really caught my eye and I just had to respond!
"The sky's the limit" means that there is no apparent limit to how far you can go in what you are doing.
In the context of poker specifically, it means that there is no maximum amount of money or chips that a player is permitted to wager.
I think that the show meant it in both ways. On the surface, it related to the poker game they were playing, but it also is a way to describe the whole show. Being on the Enterprise is such an adventure that "the sky's the limit" - there's no limit to the kinds of experiences they can have on the ship.
Good question and I hope I helped!
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