10 internal users are allowed. But if you want to test more users, just run the test locally on your desktop?
Fredrik, The max number of named users in a free edition (sandbox) is 10. This includes local users, app cloud users and demo users. So when you add 5 local users, you probably access the app with your own account (this happens automatically when you deploy it from the modeler and click view app in the modeler) and have 2 demo users you have already 8 users. I guess you have 4 demo users, that would make 10.