Heinz Ketchup is a good example of why a hashgocha can certify so many products. It is essentially something which needs little supervision; as long as they are consistent in the ingredients they use (they presumably are because they want every bottle of ketchup to taste the same) and the OU has ensured that they are fine, there is no requirement for a full time mashgiach. They probably don’t use that equipment for anything other than ketchup.
Whether or not ant particular hashgocha is stretched too thin is definitely an important consideration; I’m merely pointing out that it is not impossible to responsibly give hashgocha on hundreds of thousands of products.