“ The same applies to four or five people; although three might be left for zimmun, the ones who separate could not say it. When there are 6 people, they can separate into two groups of three and say zimmun.
Similarly, since ten people are required for the use of God's name in the zimmun [see previous mishnah] , any group of ten or more people cannot separate, until there are twenty, in which case two groups of ten can recite. ”