Nope. Concurrent user limits were never enforced, and now, the licensing has changed to a per-seat (aka 'named user') model, or a 'population pricing' model.
In the old days, if IBM caught you in this situation, they'd send you a bill for the additional licenses.
If you only have a license for 10 users, and there are 15 on the server, you definitely need to figure out if those users are properly licensed. Contact your management for clarification.
-JD.