Sam4: The Gemara (I believe Menachos 29b) says that a Kiyor has to be big enough to hold enough water for 4 people, because the Pasuk says “Verachatzu Mimenu Moshe Aharon U’vanav”, which is for people. The Shittah Mekubetzes there (I believe Os Gimmel) asks that there was never a time when that is only 4 people and we should learn that 6 is the minimum. He answers that it must have been on the 8th day of Milu’im. When the Gemara says that Moshe only did 7 days it must mean that he only did 7 full days (my Teretz).
