I have a minimum standard of 1 vCPU / 4GB RAM / 60GB disk for my Windows 2008 VMs, but if I think the application will call for more resources, I'll add a vCPU and some memory. I also tend to look at the vendor's hardware recommendations; In particular Microsoft has been doing a decent job lately of laying these out for their roles and different server apps because they have a vested interest in virtualization these days as well. So it would be beneficial to see what Technet has to say on recommendations for the number of users and the roles you need to implement, in addition to the software vendor for the client apps that will be run on the Terminal Server. If very many concurrent users are going to be using the server, 2 vCPU/8GB could be a bit low.
The other end of the spectrum is when people request 4 or 8 CPUs and 32GB RAM, "just in case they need it". If I can determine that is the case, they get what they get (much less resources) and I'll add more later if the situation warrants it.