Like Undisclosed 1 says, it really depends on the VMS. It can't be simplified to a certain number of cameras needing a certain processor/RAM combo.
Moreover, you want the manufacturer to fully support the system, so asking and following their recommendations is important. If you don't follow them, they could simply come back to you should issues arise and tell you the server was under specified.
All that being said, I can sum up some recommendations from our VMS Server Load Fundamentals:
- You're better off spending money on increased CPU than increased RAM. Memory is rarely a limiting factor in throughput handling on a VMS server. If there is a lot of processing happening, like advanced analytics or server side motion detection, it may have an effect, but even this varied depending on the VMS.
- Running server side VMD can max a CPU very quickly. For example, in our tests, server side VMD on 10-12 cameras added 20% or more CPU usage. This is why camera side VMD is as prevalent as it is.
- Finally, unless you're running very low camera counts and relatively low resolution, don't view video on the VMS machine. Viewing only 4-5 1080p cameras can drive a low end PC to 100%.