It appears that Jenkins pipelines have no way of throttling parallel closures execution.

Given the example:

stage('Test') {
  steps {
    script {
      testing_closures = [one: { print("starting one"); sleep 10; print("finishing one") },
                          two: { print("starting two"); sleep 10; print("finishing two") },
                          three: { print("starting three"); sleep 10; print("finishing three") },
                          four: { print("starting four"); sleep 10; print("finishing four") },
                          five: { print("starting five"); sleep 10; print("finishing five") },
                          six: { print("starting six"); sleep 10; print("finishing six") }]
      parallel(testing_closures)
    }
  }
}

The main goal is to throttle those closures. If we don't want for all six of them to run concurrently, but let's say only 3 at a time, we'd need an argument for 'parallel' step:
parallel(closures: testing_closures, maxThreadCount: 3)

Currently, as far as I know, there are no workarounds for this issue. If JENKINS-46235 is resolved, the following workaround could be used:

1. Dynamically create 3 lockable resources via LockableResourcesManager::createResourceWithLabel() with build-unique labels
2. Lock them by label in all of the closures
3. The closures will wait for each other to finish and only 3 at the time would be running.
4. Delete the closures (no such API method. JENKINS-46235 is for exposing 'delete' method for lockable resource)