The McDonald's signs that used refer to "Over _ _ Billion Served" were actually referring to hamburgers. It is a common misconception that the signs referred to customers served. The original sign in front of Ray Kroc's first McDonald's said "McDonald's Hamburgers Over 1 million served" McDonald's has also stated many times that the number was meant to refer to hamburger patties and not customers. Therefore, every time you order a Big Mac it is counted as two, since Big Mac's have two patties.
Ray Kroc once explained how he came to that number. He claimed that they kept track of how many pounds of meat was shipped out to all the McDonald's locations in total, so they just divided that weight by the weight of an individual patty, to get the number of patties served that year.