New RS485 driver chips are said to support 128 or 256 slaves.
RS485 is specified as handling 32 "unit loads". RS485 transcievers are available in 1, 1/2, 1/4 and 1/8 unit load. As long as you do not exceed 32 unit loads on your network, you can mix and match.

The problem you are going to have with a rig like this is to get the terminations at the right places, this is the one thing any 'problem solver' that see your rig WILL screw up.

Do not miss the goodies in "ten ways to bulletproof rs485", it is found under 'best serial appnote site' in links <---. Following each and every rule in that note does, indeed, provide bulletproof RS485. For EMI reasons, you may want to use reduced data rate chips (even 1Mb is considered a reduced data rate), they round the pulses a bit and thus cut the radiation. I always choose chips reduced to twice the rate I use.

Erik