I guess by 'concurrent sessions' you mean idividual internet sessions, ie 1 person downloading a file, one person checking mail etc.
Astaro is limited by hardware, license and your bandwidth. Throw a high end P4 processor and gigabit NIC's and an Enterprise license at it and I don't doubt you could handle hundreds of users.
there is a Performance Brief available for the Astaro firewall. It's not Up2Date (for Version 2) but there you can see the dimension we're talking from. Meanwhile you would use faster processors etc. so throughput factors should be multiplied [:)]