Thank you for posting.
The full FileMaker Server EULA can be found here:
In general, the license allows an installation of the FileMaker Server software with the following restriction regarding FileMaker Pro client connections:
(a) Maximum Number. The "Maximum Number" of clients or sessions as used
in this Section 2 is stated in your Software documentation and in the "Clients" tab of the
FileMaker Server Admin Software.
(b) FileMaker Pro Clients. You may not allow more that the Maximum Number
of FileMaker Pro clients to concurrently access the Software. In addition, this License does
not grant you any rights to use FileMaker Pro software, and you must separately pay for
and license each copy of FileMaker Pro that is used with the Software.
A Client Access License (CAL) is a license for connecting to Windows Server 2008. They are independent of FileMaker software and I cannot say whether they would be necessary for FileMaker Pro clients or not. Besides the CAL, you were correct in your assertion that you can have 250 connections to FileMaker Server assuming you have a license for each FileMaker Pro client.
You can find more information regarding CALs on Microsoft's site:
CAL's are not needed for FileMaker--at least we didn't need to purchase one. They can be useful in setting up domains for better management of your system but just hosting FileMaker on the machine doesn't not require buying additional CAL's.