We're building a new application for a client and their question is how many users can DotNetNuke have? What is the true scalability factor with DNN? Thank you for your help in advance.

I think the theoretical limit of users in a single DNN install would be 2,147,483,647 (minus a couple for Host and Admin). This is based off of the INT data type for the UserID in the database.

I don't run any sites big enough to warrent a web farm, so I don't deal with anything on that level. I think that the first stumbling block to a sites growth to that level will be hardware and distribution set up.

Next would probably the quality of the module(s) you are using. If you do most things in house / get source, you will be able to work on that much more effectively than a request-test loop with the original developers of purchased modules.

This is all me speculating. Since I, personally, haven't encountered these issues, but I do like to think about these types of issues. :)