You can put configuration limits on the number of allowed sessions. See <max_clients> on the Server configuration file.
You can also make the Server disallow new sessions based on a heuristic measure of the machine load. See <max_load> on the Server configuration file.
In both cases, the limit is on the sessions, but there is no limit on the subscription requests related to each session (that is, control connections are not limited); if you need to put limits on the activity of each session, you should do it through the Metadata Adapter.
For general considerations about Server performances, see
this FAQ.
