Per-'direction'. ie: if you fly (YVR-YYZ-YUL)-(YUL-YYZ-YVR), you will be charged the fee twice as there are two directions. (YVR-YYZ-YUL)-(YUL-YOW)-(YOW-YVR) would incur 3 $10 fees.
Easiest way to think of it is that for every fare you 'click' on the AC.com booking site, you will be charged a $10 fee to select a seat.