The answer depends on whether the area of the intended swing is improved. In some cases, the knocking down of a number of leaves would not improve the area of the intended swing as the player still has to swing through a number of remaining leaves when making his stroke.
So I've knocked down 3 leaves but there are 4 remaining and that's OK even though it is harder to swing through 7 leaves rather than 4 - barmy!!
The decision quoted (13-2/22 Knocking Down Leaves with Practice Swing) goes on to say:
"In other cases, the knocking down of one leaf might improve the area of the intended swing, in which case there would be a breach of Rule 13-2."
So if it is easier to swing through 4 leaves than 7 the area of your swing has been improved and the penalty applies. If it was 3 out of 300 then maybe no penalty.
I think you have to make the decision before playing as Decision 13-2/22 also says
"If a player has improved the area of his intended swing by knocking down a leaf or a number of leaves, he cannot avoid penalty under Rule 13-2 by subsequently changing the area of his swing when he actually makes the stroke."
There isn't a definitive answer or formula as to how many leaves or how you decide, you have to assess each situation. If in doubt, in equity, take the penalty, which I guess is what upsidedown's opponent did.
The fact that Rule 13-2 has 37 decisions associated with it suggests it isn't always an easy rule to interpret.
