Exodus 12:4

And if the household be too little for the lamb, let him and his neighbor next unto his house take it according to the number of the souls; every man according to his eating shall make your count for the lamb. If the number of members in anyone household, including the children and the servants, was so small as to make their disposing of an entire lamb improbable, then two small families having about the same number of souls might unite. Custom afterwards fixed the number of participants at the meal at about ten to twelve, but the fundamental unit was the family.