Honestly, the number of musicians hired for any given Broadway show is not a matter of conjecture or speculation. Look at the Playbill and count how many musicians are listed. That's the number of musicians and the instrumentation that will be playing there that day, unless someone is missing due to an emergency. (Rare, but it does happen.) It might not be those exact same guys, but it will be that exact same number.
No producer or music director ever says "let's just throw in another oboe and trumpet player today just for the heck of it."
There are cases where a show will hire more than the minimum number of musicians initially but have a "cut list" of players who may be fired at a later date. This is allowed as long as the band doesn't go below the house minimum and the musicians know they are on the cut list from the beginning. Both Ragtime and Phantom did this. I've never heard of extra players being hired in the course of a show's run, but perhaps it has happened. I'd be interested to hear about it.
"I have got to have some professional music!" - Big Edie