How many standard cake boxes will i need if i'm baking 3 or 4 9 inch round cakes?

One standard box of cake mix (approximately 15-19 oz.) will make two 9" layers. They will be on the thin side. Which ever cake mix you are looking at should tell you what size layers and how many it will make.

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.

In a 9 1/2 inch pan one box with an extra egg was perfect for one pan. I would get 4 or 5 to be safe, better than having to go back to the store.