Originally Posted by WestJet

Hey armeee,

If you choose to reserve your seats at the time of booking, the fee is payable per flight.

It seems like you're flying Ottawa to Las Vegas with a connection in Toronto, so here's how the fees would break down:
Ottawa - Toronto $5.65
Toronto - Las Vegas $21
Las Vegas - Toronto $21
Toronto - Ottawa $5.65

You don't need to reserve your seats for the entire trip if you don't want. For example, if you're not bothered about where you sit on the short-haul flight, but you do want to make sure you're seated in your preferred seat on the long-haul flight, then that's OK.

Hope this helps,
Darren