It depends on the instruments, but I would have thought 40. But you should put in 4.5GB, in which case you'll be able to load about twice that.
Don't put a matrix reverb on every track. Use one shared Space Designer convolution reverb, and put a panner (I forget what they call it) on the send of each instrument to position it in the reverb mix.
Don't put a matrix reverb on every track. Use one shared Space Designer convolution reverb, and put a panner (I forget what they call it) on the send of each instrument to position it in the reverb mix.