The max current sessions is based on your license. So if you have a license for 50k sessions you can manage 50k unique sessions.
Concurrency can be set a few different ways. If you need to enforce concurrency you can do that by setting Apply-Login-Limits=yes in the radius.ini and then sending the Login-Limit attribute in a return list from a profile. The Login-Limit attribute defines the number of allowed concurrent sessions. If you aren't concerned with concurrency you can turn it off globally by setting Apply-Login-Limits=no.
You can also use the %Login-Limit to map an LDAP or SQL attribute to the concurrency limit for SBR.
You should know that SBR Carrier has more advanced concurrency functionality that is a licensed feature.