Ostensibly as many as you like, although some sellers may limit the number of items you can buy. The best person to ask in a situation like this is the seller. Tell the person how many of which item you would like to buy, ask if that is okay and what it would cost to mail them all to you.
If the seller is in Canada it should all be really easy, but if the seller is in, say, the US, then depending on what the items are and how much the lot weighs it can come either First Class International (the most affordable) or Priority (more expensive). There is a 4 lb limit on items coming First Class, and Priority is tracked. So it's the seller's choice. Just be sure to get your shipping quote before you bid or buy.
And I'm sure you already know, - items over $20 may incur duties and taxes coming over the border. Canada post charges $8.50, but couriers, especially UPS charge huge fees for the service. Meaning that cheap $10 'UPS Standard to Canada' fee you see will cost you so much more when the stuff arrives.
For now though, - ask the seller.
