When logging into WordPress with a user account, WordPress checks your credentials against those stored in the database. If they are correct, it then sets a session variable in your browser and you are able to view all areas that check against this session variable to gain access to.
Sessions are set on a per browser basis so it does not matter how many browsers are accessing the site as they will each have this session created on their own browser if their credentials successfully authenticate.
You would only run into problems if for example multiple users were trying to write to the database at the same time for example but in your case there is no admin access so for viewing purposes only, not problem.
Hope this explanation makes sense 🙂