The raised number was designed to mark the spot where you should tap in order to get ketchup onto your food.

 A spokesperson for Heinz said, “To release ketchup faster from a glass bottle, here is a little secret from Heinz. The sweet spot to tap on the Heinz bottle is the embossed 57 on the neck. All you need to do is apply a firm tap where the bottle narrows and the ketchup will come out easier.”

[Source]