I seems intuitive that it would add two.
First, one would assume that On Collision is a triggered condition, as most conditions begin with the word "On" are.
A green arrow appears before the condition name on the event sheet for triggered conditions:
<img src="http://dl.dropbox.com/u/1013446/forumhelp/trigger.PNG" border="0" />
in the screenshot below you can see this is not present on the On Collision condition.
Since the condition is not triggered by each collision, and instead is evaluated once in the order it appears in the event sheet. Those actions are carried out one time, regardless of the number of colliding instances, and all instances are picked at the same time. This is useful, because you can do things like make a sound play once even in the event of multiple collisions, instead of playing twice at the same time, etc,
If you need an action for each collision, you can still use a 'System: For Each Object' subevent to loop through each of the objects that are picked in the collision event - which results in the behavior you originally intended:
<img src="http://dl.dropbox.com/u/1013446/forumhelp/bricks.PNG" border="0" />