In-app security we don’t configure an anonymous user, we actually configure anonymous user role and then assign it to a particular role, in each module.
Hope it clears your confusion!
take a look at this : https://docs.mendix.com/refguide/anonymous-users/
"You can use anonymous users to allow end-users access your application without having to sign in”.
So there is no "how many", as everyone entering your app have this role before signing in.