Thank you so much for reading the Access 2010 programmer's reference and for posting your questions about ADPs! I'm so sorry it has taken me so long to get back to you about this, I'm just seeing it now, but hopefully I can help out here!
In general, limitations for ADP data connections and users are specific to SQL Server (because the ADP is just using ADO under the covers to access the data on the SQL Server) and what you are saying here is correct. The Microsoft Access team provides a bunch of information about ADPs and their limitations at:
Under the section about half way down on the page, titled "Microsoft Access project specifications", but this doesn't really talk about the number of users, again, because this is dependent upon SQL server.
In general, the recommendation is that every user use a separate copy of the ADP to connect to the SQL server (i.e. not 50 users accessing the same ADP file simultaneously...each user should have a separate copy of the same, or different, ADP file to run the application) and in theory, you should be able to have thousands of users working with the data simultaneously. Does this make sense?
Anyway, I hope this helps answer your questions about ADPs in the multi-user environment, but if you have other questions about this, please let me know and I'll do what I can to help!