Andrea,
Do you already have a VSM7 Operations Manager (VSOM) license?
# of cameras really depend on the design (bitrate, location, etc.). The licenses are installed via VSOM, not Media Server.
Each non-Cisco IP camera will need a license. Same part number, just buy the quantity that you need. Licenses can be added anytime. There is no need to order licenses for CISCO endpoints.
I believe each media server will handle around 200-250 cameras. Again, this all depends on what the camera(s) are streaming, etc. Also will depend on the server platform. On UCS-C, it'll also depend on how many VSMS virtual machines you are running.
Hope this helps!