Ohh! Got it 
Yes, a chat application is completely possible with Bubble! There is no limit on the amount of Users who can be logged in at the same time, and there is no limit to the amount of Users who can sign up for your app (a new User account ‘Thing’ is added to the database each time a User signs up for your app).
With Bubble, the more Users who are logged in at the same time, the more data searches and actions are running at the same time, the more capacity is being used, and that then affects the monthly price of that Bubble app, as your app scales. So, if your app is very popular and it is hitting max capacity consistently, that just means its time to add capacity (or restructure some data searches and workflows so they use less capacity, if possible). Here is more information:
In summary though, there is no limit
*In the opposite example, you would leave the login workflows as they are, and they will just continue to log Users into that same account without logging anyone out.