I guess it obviously depends entirely on the prospective clientelle.
I don't think TOO many pianists would be willing to wheel in their Bossendorfer for a quick overdub for example, but on the other hand, I do believe that there are two VERY BASIC principles relating to a studio providing instruments.
The first is obviously every GEARSLUT'S favorite, and that would be to provide a varied pallette in the forms of multiple snare drums, cymbals, guitar amps etc etc as it may well count as one of the plus points for artists choosing a studio ... I mean, a lot of musicians are also gearslutz at heart and LOVE scrounging through the storeroom and digging out the old AC30 etc.
The second, which I THINK is perhaps more related to what you were initially asking, is to provide at least one QUALITY instrument to replace the 3-stringed no name brand guitar that the misty eyed pratt (who just so happens to be paying for the session) is weilding.
On that level ... I would (PERSONALLY) say that ONE good guitar, bass, acoustic guitar, guitar amp, perhaps a bass amp, drumkit, one or two additional snares (piccolo for example), cymbals, and SOME form of universal keboard arrangement (my opinion on this is a nice weighted controller, a non-weighted controller and an old fashioned sampler with a healthy sample collection) is a solid starting point, although many may beg to differ.
Of course there are exceptions to the above .... (I don't run a big commercial facility, but have worked in MANY) .... and I would think that one possibility is providing as many "familiar" instruments as is possible ... that is a big issue with electronic instruments, where a player may be familiar with a minimoog for example and won't want to mess about figuring out the Waldorf Wave for the entire morning's session ..... but I think that this train of thought is tightly related to my first scenario as above.
Just my opinion.
Ciao