How to control ParallelSuitethread count?

I have following code:

@RunWith(ParallelSuite.class)
@Suite.SuiteClasses({Test1.class,
        Test2.class,
        Test3.class,
        Test4.class,
        Test5.class,
        Test6.class,
        Test7.class})
public class ParallelRunner {
}

I ran it and figured out that only 3 test run in parallel. These test anough long - 15+ seconds

Is it OS scheduler issue or junit thread pool limit?

How can I configure thread pool limit ?