SQL Server Admin 2014 :: Practical Upper Limit On Number Of DB Users

Our development team wanted to create a database user for each application user in the application and use these for granular data access control, which at first, sounded like a good idea but our initial testing ran into some interesting results.

Our target user base was about 15 million users with an estimated 1% concurrency rate, and finding no MS documentation on an upper limit to the number of users a database can have we began some load testing to see how the database performed. In the hundreds of thousands of users range our test database had a hard time performing well under light loads (even without any concurrent connections).

When we purged the users and reverted back to just a handful of service accounts, performance went back to "normal" under the same loads. I began to wonder if this is a situation where throwing more hardware at the problem would overcome the issue or if there is a practical upper limit to the number of users a single database can handle well.

(There were of course other cons to this arrangement and I certainly was never going to expand the users tree in the object explorer for a database like this, but we thought it a solution worth investigating.)

What is the largest number of users any of you have had in a single database?