Is there a limit on the number of logins allowed using the same username?
There is no limit on the number of people that can be logged into one account at the same time.
The only concern would be if they are allowed to edit things, there is the chance that views and files can be overwritten. (User1 logs in, changes a view, and saves it. Then User2 logs in and changes the same view. The view will be seen as User2's since they are using the same account)