There's a boatload of variables that make a difference on how many you use in a project.
Type of wood
Wood thickness
Blade size
Blade tension
Feed speed
There are a lot more. Blades do break fairly often, but if they are breaking excessively, I would look at a couple things. First and foremost is how hard you're pushing the work into the blade. You shouldn't push any harder than it takes to cut without moving the blade back more than a tiny bit. It's hard to describe without seeing it, but you need to let the blade cut and move the work only to follow the line without trying to make the blade cut faster.
Another thing is blade tension. Too loose will cause blades to break quicker as well as too tight. Steve has a video somewhere in his collection that shows the sound a blade should make when plucked like a guitar string.
Blade size is another one. Too small a blade on too thick of a piece can break them quicker.
Personally, I do better with small blades I rarely use anything bigger than a #5 whether it's intarsia or fretwork. On 1/8" or 1/4" wood, I use #3 and #1 a lot.
The project also makes a difference.
If I use Steve's Panther pattern that he posted this morning as an example, that would easily be a very quick and simple one blade cut for me. Most of the time that is. Occasionally I might get a bad blade, but I would say that cutting that panther out of 1/4" wood I would use a #3 ultra reverse and 8 out of 10 times, one blade would do it. Probably a #5 on 3/4" stock.
Oh and if you don't already do it, packing tape on top of the pattern also makes blades last longer!
Ray