The default WiredTiger session max is 33000. This level of concurrency is unheard of, and nearly impossible due to the ticket mechanism. There are two main challenges, however:
1. There is no limit on idle multi-doc transactions.
2. If we hit the session limit, internal operations will start failing, which will affect system availability.

The first problem should be addressed in SERVER-108215
The second problem should be addressed in this ticket. Impose soft-limits on user sessions and reserve a portion for internal operations.