When I worked in television, in our market the typical rating for "Star Trek: The Next Generation" was a 4. However, "The Cosby Show" (a big hit at the time) typically rated a 19. This said to me that the number of Trek fanatics was relatively small in relation to the general population.
TNG was consistently the #1 rated one-hour drama in first-run syndication. Sounds good. But it wasn't the #1 show in first-run syndication; that honour belonged to "Wheel of Fortune", with Vanna White, IIRC. And TNG certainly wasn't the #1 show in first-run network TV. "The Cosby Show", according to Wikipedia and
TV Guide, "was TV's biggest hit in the 1980s, and almost single-handedly revived the sitcom genre and NBC's ratings fortunes".
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Having said that, there was a man-on-the-street poll conducted during TNG's TV run (mentioned in a "Best of Trek" volume) and a surprising large proportion of the US general public admitted to considering themselves a fan of Star Trek. Of course, that doesn't mean they watched every episode in first run, or bought any merchandise.
Analysis of cinema audiences for films like ST IV, compared with the rise of ST tie-in merchandise throughout the 80s and early 90s told Paramount that about 1-2% of cinema audiences were buying the licensed novels and comics. And, IIRC, about 10% of those audiences claimed to be avid Star Trek fans.
The ten percent rule seems to work pretty well, even here on TrekBBS. Surveys before JJ's film premiered had about 10% claiming it would flop. It seems that now it's out, about 10% are disappointed enough in it to keep grumbling.
If JJ has satisfied 90% of pre-existing fandom, that's probably as good, or better, as we could ever have expected.