How many standard cake boxes will i need if i'm baking 3 or 4 9 inch round cakes?
If you are in the US, a standard box of cake mix makes two round layers. So you will need 2 boxes to make 4 layers. Outside the US, I think the same applies, just read the box.

I recommend 3 layers if this is among the first few cakes you have made. Bake the two boxes (4 rounds) then choose the best three of the four rounds out of the oven.