Glassfish limited to two simultaneous threads with JPA/EJB/web service

Answer 1

I doubt there is such limitation. Perhaps the job was finished quickly, combined with connection delays, and hence the entity managers were reused.

In order to make a better benchmark, use JMeter - fire 15 requests, each in X seconds, and share the output. (this article gives a start for using JMeter. But it's very easy anyway)

Answer 2

Check configuration>thread-pools>thread-pool#min-thread-pool-size in the admin console (should be 2). But that's just a min, certainly not a max.