I've fallen into the one whistle blast hit for foul, two for violations like travel / carry / double dribble etc.
One whistle for subs only if I am the traffic cop
One whistle putting ball in play if we've had a delay for some reason - subs taking longer, discussion at table or whatever.
Scrum on floor - jump ball - LOTS of whistles.
Speaking of whistles -
http://www.youtube.com/watch?v=ZYgrhk7xoME