Eruvin, Chapter 7, Mishnah 8. This mishnah deals with the measure of the eruv: what quantity is required for eruvei chatzeiros (joining of courtyards) and for shitufei mevo'os (joining of alleyways). The mishnah opens with the question: "Kamah hu shiuro?" - how much is its measure?
"Bizman shehen merubin" - when they are many:
When the residents of the alleyway or of the courtyard are many - and according to the Gemara this means eighteen people or more - the measure is "mazon shtei se'udos lechulan" - food for two meals for all of them. A quantity of food sufficient for two meals is enough for them all:
The measure of the two meals: eighteen dried figs (grogaros), which is about six medium eggs.
For each meal: three eggs.
"Bizman shehen mu'atin" - when they are few:
When they are fewer than eighteen, the measure is calculated as "kigrogeres latzeis haShabbos lechol echad ve'echad" - the measure of a dried fig for each and every one, the same measure that determines liability for carrying out on Shabbos, since a person is not liable for carrying out food unless it is the volume of a dried fig. This measure is counted for each individual.
Once their number reaches eighteen, the measure has reached its maximum. From that point on, eighteen dried figs - about six eggs - suffice for any number of people, since two meals are a significant quantity that is enough for everyone.
An additional interpretation of "lechol echad ve'echad":
Some understand that the closing words "lechol echad ve'echad" do not refer to each individual person, that is, we do not count by the number of heads, but rather by the number of participating units: for shitufei mevo'os, according to the number of courtyards, so that each and every courtyard requires that same quantity, and if there are eighteen courtyards, eighteen dried figs suffice; and for eruvei chatzeiros, according to the number of houses, and not according to the number of individuals.
In summary: in this mishnah we learned the measure of the eruv - when they are many (eighteen or more), food for two meals for all of them, which is eighteen dried figs, about six eggs; and when they are few, the volume of a dried fig for each and every one, with eighteen dried figs being the maximum measure for any number at all. We also noted the additional interpretation, that the counting is done according to the courtyards or according to the houses, and not according to the individuals.