Answer:
here's an unvetted list as per 10/2010
approx 450
Geo-sync orbits are hard to maintain as the orbit decays almost immediately upon achieving it, lending to low service life span thus the list changes a lot
positions are usually chosen on the 1/10 of a degree of the planets circumference so 360 x 10 is the ideal maximum amount of sats in this orbit at one time.
there are slightly off orbits named super and sub geosynchronous which move slowly across the orbit either westerly or easterly, usually marked for junk satellites or discards, some electronic lab platforms have been put here however, the diameter of these orbits very and are often highly elliptical so pin pointing these related orbits and how many positions they offer is unlikely to be meaningful