How to understand forkjoinpool parallelism?

Question

public class Demo {
    public static void main(String[] args) {
        ForkJoinPool forkJoinPool = new ForkJoinPool(17);
        for (int i = 0; i < 100; i++) {
            forkJoinPool.execute(() -> {
                System.out.println(Thread.currentThread().getName());
            });
        }
        try {
            TimeUnit.DAYS.sleep(1L);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

output (part of console)

ForkJoinPool-1-worker-4
ForkJoinPool-1-worker-19
ForkJoinPool-1-worker-18
ForkJoinPool-1-worker-30
ForkJoinPool-1-worker-1
ForkJoinPool-1-worker-23
ForkJoinPool-1-worker-8
ForkJoinPool-1-worker-16
ForkJoinPool-1-worker-25
ForkJoinPool-1-worker-11
ForkJoinPool-1-worker-29
ForkJoinPool-1-worker-9
ForkJoinPool-1-worker-5
ForkJoinPool-1-worker-15
ForkJoinPool-1-worker-22
ForkJoinPool-1-worker-26
ForkJoinPool-1-worker-12

Why more than 17 threads?

In my test case, I want set 17 threads, how can I do that? How to understand the parallelism of forkjoinpool's constructor?