It's usually one bottle per toner cartridge. I've found for the most part, one bottle is more than enough and I usually have a little bit left over.
You might want to do some research into how easy (or hard) it is to refill the toner. Some refill kits want you to drill a hole in your cartridge so you have somewhere to fill the cartridge. Some people may not be comfortable with doing so.
I have no affiliation with these guys - they were just the first result when I Googled Vancouver toner refill (
http://www.inktonermart.com/hp-q2.html) and they want $39.99 for a refilled toner cartridge. If you don't like the idea of being without toner, just buy a refilled cartridge and then keep your empty around and order the refill kit from wherever you like and keep it aside for later use. (I think DealExtreme also sells the toner refill for pretty cheap as well)
One "trick" you can try if it's printing unevenly is to take the toner cartridge out of the printer and shake it side to side to redistribute the toner within the cartridge. You can probably get a few more pages out of the toner that way.