The number of devices serviced by a single server is only limited by the available server resources, mostly CPU power and RAM size. It will greatly vary depending on the number of device metrics, their polling/update period and storage settings, as well as the devices' event generation rate. A detailed server hardware selection guide is available here.
OEMs, VARs and system integrators designing AggreGate-based solutions should consider the performance optimization hints provided independently for each type of an "active" server object. Check the system documentation for details.