Brian just posted a really long post about some work we're doing to estimate how many users you can support on a Team Foundation Server.

The quick summary - we've been too conservative so far, and it looks like we'll actually be able to increase the number of users that we say we can support in v1 for all our configurations. More than just telling you the numbers he also talks a lot about the philosophy and process we've been using to estimate real world usage.

I don't know if he mentions it specifically, but I should also call out that we are using the VSTS Load Testing tools in our test runs to drive the system - yet more dogfooding.