The number of users logged into the system is not the number of concurrency user. You should focus on the actual users processing /querying.
With the assumption that:
You will rarely see concurrency go above 20% of logged user unless its specific to certain business processes (for instance Financial Management during a financial close period).
You will have with a total of 1000 users:
Number of concurrent users = 1000 x 0.2 x 0.2 = 40
These are general rule of thumb/guidelines. It is very difficult to suggest capacity planning recommendations without analysis of the expected usage.