Do you need two rubberbands on top when you are making a hexafish rainbow loom?

You need three, if you are referring to before you loop over the bottom band. After that has been looped, there will be two, so you must add another ban to make it 3. You then loop over the bottom band, add another, continue this process until it's big enough. The ending is quite difficult to describe, so I recommend Justin's Toys' tutorial on Youtube.