Basically, the more, the better. However, GigaSampler/Studio works mostly off your hard drive, so you need a fast hard drive (7200 rpm or higher) to access multiple or large samples.
GigaSampler/Studio only loads 64k of each sample into RAM, but this is of each sample of a multi-sample or multi-layer instrument. For example, GigaPiano uses about 40mb of RAM, so if you do the math, that's ALOT of individual samples!