An earlier thread hereabouts (http://forums.microsoft.com/msdn/ShowPost.aspx PostID=2316) implies that version 1 of Team Foundation might struggle with a huge number of concurrent users/and or files, and that version 2 will "include all the scale work to run servers as big as we need them".
I've seen a limit for number of users in version 1 suggested as 500 (http://forums.microsoft.com/msdn/ShowPost.aspx PostID=8764). Any limit on the number of files (Assuming the SQL Server machine has sufficient disk space to cope with them all, obviously.)
I'm working on a project which will use Team Foundation as a document repository (i.e. with a custom front-end using the classes within the Microsoft.VisualStudio.Hatteras.Client namespace) and it will end up needing to store hundreds of thousands of files (or versions thereof). Will version 1 be up to that
I can always use multiple repositories and have the client abstract that fact away from the user, but I'd be interested in knowing what a realistic per-repository limit might be.
Cheers, Ian.

Maximum Number of Files in Repository?
Prayag Gandhi
I believe our internal dogfood host is already holding upwards of a million versions of items. I don't have the number of items off hand (someone who sees the data more regularly will hopefully chime in) but I know it's into the hundreds of thousands.
jirkan
I just surveyed our dogfood server (you can query your own server by visiting this link: http://localhost:8080/SCC/application/Administration.asmx/QueryRepositoryInformation)
Users: 409
Workspaces: 905
Shelvesets: 1791
Files: 530,134
Folders: 63,894
-jeff