If you are not getting enough instruments:

• Make sure you have enough RAM
• Make sure the "Auto-convert Samples to 32 bit" button is unchecked in the options menu or this will double the size of the samples
• Make sure to use only the "dry" instruments.
• If using Ambience, if you lower the quality knob you can regulate the cpu usage and get better performance.
• Make sure you adjust the buffers on your soundcard.

(from garritan.com)