It depends on the show like Ed said.
There may be more musicians that are in a different part of the building that are mic'd. Wicked [Chicago] has the percussion player [not the drummer] in a separate part of the building.
Depending on what instruments you're referring to, some of them may be synthesizer [not sure on the regulations of that though].
In a show such as Jersey Boys or The Wedding Singer, there are additional musicians in the pit to what you see on stage.
E.g. for The Wedding Singer there are 4 guitarists listed, 3 of which I believe were in the pit, 2 bassists-one on stage and one in the pit. The guys on stage play for certain parts of the show, but their main role [in most cases] is as an actor.
If you check IBDB.com you can see the listings for wide range of shows that are in New York.
For a great look at theater playing check out
www.bassplayer.tv and click on Artist Interviews and watch the videos by John Miller.