More information, McAfee Endpoint Encryption for PC 5.2.11.
lots, thousands. It depends on how big you make SBFS, and what features you enable - typical user size is around 30Kb though.
the client log will tell you how much free space you have. You can stretch SBFS up to hundreds of MB though.
more users means more sync time, and a greater possibility of an account with a default password on the machine. I would say keep it to less than a few hundred.