Ok here is what you need for starters:
If you are buying 1 NEW tank, you will need 1 controller. If you are buying two tanks but only you will be using them you can still only have 1 controller. The ROM chip that comes with the tank is interchangeable between the controllers so you only need 1 controller for 4 tanks. But the downside is you wont be able to play with anyone. If you want to you need at least 2 tanks and 2 controllers. Thats all you need to get started. Then once your situated you can invest in a 4 bay charger, Sound system and 4 chips such as I
Good Luck