BE Server license

Hi, our customer plans to buy BE. They have 21 Windows servers (including the backup server) in total. They need to back up the filesystem of all these servers. They want to know how many licenses they need to buy.

My question is:

With the BE server license, can BE server back up itself? If yes, does it mean in this case they just need to buy 20 agent for Windows licenses + 1  BE server license?

Thanks in advance.