You should consider about simultaneous connections only. Because there is no conception for simultaneous uses. If you use Workgroup standard license, you can use SQL Server Express only, and simultaneous connections will also limit to 10. But make sure that "limit to 10" will only restrict for client server system such as ArcGIS Desktop, not for the Web Services. I guess your clients will connect to RDBMS through Web Services. So you can connect unlimited. However, SQL Server Express is designed for small scale. If your system will be a large scale, I recommend you to use Enterprise Standard License.
For more information, following this documentation.
In "ArcSDE geodatabase options" topic, there is
"When licensed through ArcGIS Server for Workgroup, 10 desktop clients and any number of Web clients can connect to the SQL Server Express instance."
Hope this helps you.