As a start-up package, the virtual machine with at least 4GB RAM and 2xCPU is required.
Disk space doesn't matter. 10 GB would be enough to setup the host OS: Ubuntu Linux 18.04. SSD drive is recommended.
This server can run up to 200 online clients with logging, and up to 1000 online clients without logging (you can still turn on logging for test configurations).
For larger amount of clients, you can use a dedicated server with 16Gb RAM and 6xCPU and 250Gb SSD drive. Such server may host up to 2K online clients with logging, and up to 10K online clients without logging.