Protocol is one to a customer. The point of inflight dining is to keep you alive, not sate you to fullness. If you need more, bring more. (Of course, if this is a US domestic flight where you pay a la carte for food items, you can buy multiple items -- though it would be very rude to buy a whole armful, thus enduring that the back ten rows get nothing. If this is an international flight with free food, you get one tray per meal.)