The 10 user restriction is imposed by the Mendix Runtime when using a free Sandbox cloud node, so in order to create your new account you need to actually delete one of the existing accounts as you discovered.
The setting for 'number of rows' you discovered for the data grid is actually just the number of rows that the grid will show in the front end per page. It doesn't have an impact on the maximum amount of data that can be stored in the database.
Good luck in your exploration of the Mendix platform!
I have imagined that would be it.
Thanks for the tip, Rob!
Hello! Where did you find the “Account_Overview” page, where you are able to modify the local users?