No, CSA licenses do not count as a device. The 20 Device limit refers to actual pieces of hardware that you manage/monitor or otherwise maintain with VMS. Firewalls, Routers, VPN Devices, etc. As far as I know you can have as many CSA agents as the server you have it loaded on will handle. There are also some restrictions based on what type of database you have on the server. SQL will let you get a lot more agents than MSDE.
The unrestricted ver$ion of VMS costs more of course but does not have 20 device limit on it.
Hope this helps.
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