Based on http://www.alloscomp.com/bitcoin/calculator.php:

1 250 000 khps will have an average generation time of 10 minutes.
  900 000 khps will have a median generation time of 10 minutes.

This thread claims that the average generation speed is about 400 khps, I'm seeing about 1000 (reasonably recent machines).  That would give an estimate between 1000-3000 machines working in the swarm.