CBSA will use their discretion to exempt Jerry can fuel under reasonable amount...which is vague. Also nowadays 1 Jerry can can holds 50+ liters!
You probably want to buy the jerry can in Canada. Walmart and Target are usually out of stock. The ones at Walmart when available are POS, there is no tube just a gun type lever. For a few dollars more Target has the exact same one with a tube. They are similarly priced here. I made the mistake of buying the 25 litre one at Walmart and it's a real challenge to pour gas in your tank.