According to OWC’s RAM page, OS 10.9 will allow 128GB RAM. Although the (8) 16GB RAM sticks needed to fill a Mac Pro will set you back $1,729. 
Anyhow, I have 24GB and even a very large Cubase project won’t take up much memory. (Moderate sample libraries - no big orchestral ones. Surround project with tons of tracks, effects, etc…) I’ve also tried loading up Pixelmator, Final Cut Pro X, Compressor, etc. all simultaneously to test ram usage and I’ve yet to come close to cracking 12GB. OS X is VERY efficient at managing memory! (which is expected, considering most of Apple’s customers buy laptops/mobile devices)