We have using SA4000 with 250 user license and made the stress test on this. We have failed to logon after 150 users has logon. The log files has shown "MaxSeesions" when the 151 user has logon.
Do you have a Max Session limit on the realm you're using? It would be under the realm's Restrictions there's a "Limits" tab where you can set a guaranteed minimum and/or maximum user count. That could also be the cause... but licensing could also be an issue (which you can see under configuration -> Licensing).