If you need more some easier to understand licensing policies... Ask your local reseller of choice. They can explain how it's licensed and what will work for your environment and hardware.
The above explanations while making sense, are also confusing if you've never used the product before. Me, I've used it for over 10 years so it's a non issue...
That said, if I can take a easier approach to it... There is always a core server license. This includes the ability to backup itself to a single drive in a tape library.
After that, you need "remote agents," for each additional server you want to backup. NOTE: There are application specific agents (e.g. Exchange, SQL, etc) and those come bundled with the "remote agents," included in the price.
To give you an example using your environment (assuming 15 servers all windows based and a single drive autloader)
1 BE server license
15 remote agents for WIndows
Or if you had an Exchange server
1 BE server license
14 remote agents for windows
1 Exchange license
Or if you had Exchange, Active Directory(which you have to have for Exchange), and a two drive library/autoloader
1 BE server license
13 remote agents for Windows
1 Exch agent
1 AD agent
1 library expansion option
again, talk to your reseller and they can help you a little clearer...