How scalable is GMS?
With its in built load sharing facilities, GMS is infinitely scalable, there is no limit to the size of the installation you can have.
However the user population that can be supported per machine will fluctuate depending on exactly what services are being used on that machine. The KB entry referenced below will help you to calculate that.
If you have a growing user population further machines can be added as that population reaches capacity for the current machine.