Translation:Mishnah/Seder Zeraim/Tractate Berakhot/Chapter 7/4

English Text
Three who ate together are not permitted to seperate. So too for four and for five. Six may separate, up to ten. Ten may not separate, until there are twenty.

Explanation:
As three people are required for zimmun (the formal introduction to birkat hamazon), they are not permitted to separate and say birkat separately, as then they would miss the opportunity for zimmun. 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.