More than you would think, consider buying it by the gallon and rolling on. If your dead set on spray paint, it still might be cheaper to buy paint by the gallon and getting a wagner.
It does take a lot. Youd have to check my build thread for exact numbers but I used something like 6 cans of primer and 12 or 13 cans of paint. Thats was for a camo paint job.
Takes way more than you think. I wish I would have sprayed my boat instead of using rattle cans. Probably spent twice as much on rattle cans than it would have cost to spray the boat.