Is there any restriction on the number of concurrent user licenses for WebCenter?
Yes, WebCenter Standard Edition has a cap for maximum concurrent licenses.
WebCenter 14.0.2 and newer
The cap for maximum concurrent user licenses is set to 50. If you install more than 50 regular licenses for concurrent usage, the licenses will be truncated to 50.
If you want to install more than 50 concurrent licenses, you will have to upgrade your WebCenter to Advanced which does not have an upper limit to the license count (concurrent usage limited by hardware configuration).
WebCenter 14.0.1 and older
The release has the cap for maximum concurrent user license set to 30. If you install more than 30 regular licenses for concurrent usage, the licenses will be truncated to 30.