Yes you will have more satelittes available at the receiver/rover in the field.  I've operated a Trimble FmXs with GLONASS since the summer of 2009.  I think the lowest satelitte counts reported on the display has been 8, with 11 to 12 the most common number seen.  With 11-12 satelittes visible in the clear you'll typically have enough even when operating close to obstructions.

Operating without GLONASS in 2007-2008 and getting real close to a grove of trees on the South side would sometimes cause the satelitte count to drop below 5 when RTK fix would be lost.  Since having GLONASS I have never lost RTK fix when coming up close to trees on the South side. 

When checking the status of a GLONASS base station you'll see anywhere from 13-17 satelittes being tracked.  For both the base and rover higher satelitte count typically gives lower VDOP and HDOP because of a more favorable geometry of the constellation.