Have a 2008 R2 - 64bit Terminal Server with 12GB of RAM.
Last year when I installed the server I bought 15 User Licenses but they sent me Seat licenses. I did not have time to change and I knew it would do for a while.
A while pasted and I had to purchase more Licenses. I purchased 25 User Licenses and installed those also.
The both appear to be installed correctly but all 15 of my seat licenses are taken up and it is not using the User licenses.