The NEC has looked at this differently over the years.
Originally there was the 6 handle rule, then they started talking about "lighting and appliance panelboards" and imposed a 2 breaker rule on them. In 2008 they removed that and we are back to 6 handles in most situations. You still need a main breaker (protected by an overcurrent protective device having a rating not greater than that of the panelboard) but 408.36 says it can be anywhere in the supply like the start of the feeder in the originating building.
At least that is how I read it
