(Any more could be considered greedy. Oh dear, it sounds like a calorie counter worked on this one.
Update: A recent Times report placed this biscuit etiquette (or biscuiquette) at 2 and a half biscuits, but IMHO leaving half a biscuit would be rude and wasteful. Maybe if there are 5 biscuits and 2 people, then maybe you can cut the spare bikky in half.)