No limit on concurrent users, but I wouldn't want to have 100 simultaneous users. It's memory constraint would cause issues there.

>My understanding was that one of the limits of SQL Server Express edition is number of concurrent users. Yet when I am looking at the page (link below) it does not seem to have this limitation. Am I missing it?
>
>http://msdn.microsoft.com/en-us/library/cc645993(v=SQL.110).aspx