How I can prevent too many users to sign up?
The plugin allows to disable new users to sign up at any moment. Go to General settings > Signup section, uncheck the "Allow new users to signup" checkbox. New users won't be able to signup to your instance until you enable it again.
Does signup users access to my admin or personal data?
Users create an account and a site and the plugin give the user an
admin access to his site. They are not superadmin like you.
Thus, he will be able to fully manage his site (edit site, his user data, add new users to manage his site...), without accessing your user data, general settings, plugins, or others users site.