I know that for me, if I am working on my own project, I always almost “over-bring” guitars to the session, just so I don’t get caught not having a certain sound I may want, but also really just to be on the safe side. Truth is, it’s more important to be “over-prepared” for someone else’s session, because that’s a situation where the “time’s money, money’s time” adage can really come down on you if you are not prepared for what they may throw at you! I usually try to feel out the musical situation before I come to any conclusions about what to bring to the studio for sure, but there’s no doubt you almost have to bring more than you’ll need to these kinds of recording dates. Of course, these days, every true blue session player has the omnipresent pedal board on hand, full of “stomp boxes” to get almost any sound you’d need, albeit artificially. I mean, there are certain sound that have simply become part of the music vocabulary, such as wah-wah, chorus, overdrive, delay and more, and I can certainly see the need to have these things if you expect to do sessions of a certain type.
I myself, prefer the purer sounds of having the correct instruments for the job, and enjoy making my choices, and then filling up my car with them. Every time I’ve brought too little and thought it was enough, I was always wrong! You have to first think about which guitar or guitars will be you true “go to” instruments, and then plan outward form there. If it’s an electric session, be sure to bring an acoustic. If it’s only lead they want, be sure to also be prepared to play a little slide guitar, if you can. These days, I also like to always include a baritone guitar as well as a 12-string in my studio arsenal, so as to cover as much of the tonal spectrum as possible. It would be hard for anyone to find fault with you for being so prepared, even if there was some crazy sound required that you couldn’t come up with! Also remember that most studios have the rack-mounted “outboard” equipment that can certainly give you any additional effects you may not have at your disposal, but may need.
So all in all, it’s very critical to have as many bases covered as you can when doing sessions, but I personally prefer going as much of the “purist’s” route as possible when it comes to what instruments to bring. I think if you can reach a nice balance between the “purist” approach and the “stomp box” approach, you could just end up being the perfect studio musician, and someone whose phone will continue to ring for more and more work! Good luck in the studio!