At the application level, there isn’t really a limit on the number of clients that an OBS can support. However, the number of active clients could be limited by the hardware. Specifically, each active backup session takes around 1MB of memory, so the number of active clients is limited by the RAM available. Also, the bandwidth availability could also place a limit on the number of active clients, as a rule of thumb, you can assume a 3:1 compression ratio when backing up files and a throughput of 950MB per hour using an ADSL with 256kbps uplink.

Anyhow, from system administration point of view, we would recommend having no more than 2000 users on a single server.