How many threads can a Java VM support?

Um, lots.

There are several parameters here. The specific VM, plus there are usually run-time parameters on the VM as well. That's somewhat driven by the operating system: what support does the underlying OS have for threads and what limitations does it put on them? If the VM actually uses OS-level threads at all, the good old red thread/green thread thing.

What "support" means is another question. If you write a Java program that is just something like

   class DieLikeADog {
         public static void main(String[] argv){
             for(;;){
                new Thread(new SomeRunaable).start();
             }
         }
    }

(and don't complain about little syntax details, I'm on my first cup of coffee) then you should certainly expect to get hundreds or thousands of threads running. But creating a Thread is relatively expensive, and scheduler overhead can get intense; it's unclear that you could have those threads do anything useful.

Update

Okay, couldn't resist. Here's my little test program, with a couple embellishments:

public class DieLikeADog {
    private static Object s = new Object();
    private static int count = 0;
    public static void main(String[] argv){
        for(;;){
            new Thread(new Runnable(){
                    public void run(){
                        synchronized(s){
                            count += 1;
                            System.err.println("New thread #"+count);
                        }
                        for(;;){
                            try {
                                Thread.sleep(1000);
                            } catch (Exception e){
                                System.err.println(e);
                            }
                        }
                    }
                }).start();
        }
    }
}

On OS/X 10.5.6 on Intel, and Java 6 5 (see co

At least on Mac OS X 10.6 32bit, there is a limit (2560) by the operating system. Check this stackoverflow thread.

Maximum number of threads depends on following things: