This is often a cause of confusion.
You add the serial number for Remote Agent for Windows Servers to the BE Media Server. This 'turns on' functionality which allows you to push the remote agent to other servers. Once it's 'turned on', you COULD use it as much as you like to push to remote servers - but that's not legal. So, you won't necessarily find a different serial no. on each remote agent. - It's up to you to keep track of how many RAWS units you purchased and how many you are using.
Similarly, there really isn't any such beast as a remote agent for Exchange or a remote agent for SQL - it's merely functionality in the BE media server that works in conjuction with the RAWS that has already been pushed to that server. Once you enter the serial no., the functionality is turned on. Again, that's why you won't find a serial no. on the remote Exchange/SQL server.