What kind of box do I need for TFS?
This or some of variation of this question is something I get often from the VSTS Sessions I do. The following blog posts (mainly from Brian Harry) should help
Team Foundation Server Capacity Planning
Scalability Update
How many users will your Team Foundation Server support?
One of the interesting factors though in the sizing turns out not to be the number of users but the number of projects that are hosted on the TSF box. This is something most companies will probably not take into account.
You really need to read Team Foundation Server Team Project Limits which helps in providing guidance on the numbers of projects (I am referring here to TFS Projects - not Visual Studio Projects)
Finally this post from Eric Lee on when to use Team Projects might also be useful