I can't say what the average is, but I'll provide some suggestions that should enable you to answer what I think is your underlying question:
How can I figure out how many more I should take?
Pick a couple of programs you're interested in applying to. Read the program of studies carefully, then check the courses you would be signing up for in your first year, to see what the prerequisites are. This will guide you in your course selection, and give you a fair view of how well prepared you are. Make sure to look at the textbooks used, to gauge the level of difficulty.
Something to keep in mind: if you can get a TAship, unless you have a fantastic scholarship for your undergrad, it would be financially advantageous to make the move to grad school earlier rather than later. (If you aren't accepted on the first try, you can always try again when you have more feathers in your cap.)