The licensing question is in regards to "per server" or "per seat". The per server indicates the number of PC's that will attach to that server, while the per seat says that I will have one CAL per PC that use any of my servers. From your description (and based upon my limited understanding of Microsoft Licensing), I believe you will need 35 CAL's either way. The advantage of per seat is if you add a second server and use the per seat license, you would still be in compliance. If you use the per server, you would have to commit x licenses to server a and y licenses to server b. If you set it up as 20 and 15, and today you have 21 workstations attach to server a, you are out of compliance with your licenses.
Others may understand this better, but this is my understanding. Dan