Eiruvin 7:8-9

Eiruvin 7:8

The volume of food necessary for the shittuf is as follows: when there are many residents (18 or more), enough food for two meals is sufficient for all of them. When there are fewer than this, the volume of a dried fig per person, which is the same volume that would incur liability for carrying on Shabbos.

Eiruvin 7:9

Rabbi Yosi said that the volume of a dried fig per person is only necessary when an eiruv or shittuf is first made but if the eiruv or shittuf subsequently diminishes, any volume is sufficient. The only reason an eiruv is necessary in courtyards when there’s a shittuf combining the courtyards is so that the children should not grow up unaware that there is such a thing.