"A man a lamb for a father's house" (Exodus 12:3). The term "lamb" includes both a kid and a young sheep, as it is said, "a sheep of the flock and a goat" (Deuteronomy 14:4). "For a father's house"—"a father's house" means nothing other than families, as it is said, "by their families, by their fathers' houses" (Numbers 1:2). Now, suppose there were ten families belonging to a single father's house; I might hear that there is one lamb for them all. Scripture says, "a lamb for a house"—[one for each household].
A Lamb for Each Household and Not One for Many Families
Yalkut Shimoni on Torah 192:2
אִישׁ שֶׂה לְבֵית אָבֹת בִּכְלַל שֶׂה גְּדִי וְטָלֶה, שֶׁנֶּאֱמַר (דברים יד, ד) "שֵׂה כְשָׂבִים וְשֵׂה עִזִּים" לְבֵית אָבֹת, אֵין לְבֵית אָבוֹת אֶלָּא מִשְׁפָּחוֹת, שֶׁנֶּאֱמַר (במדבר א, ב) "לְמִשְׁפְּחֹתָם לְבֵית אֲבֹתָם", וַהֲרֵי שֶׁהָיוּ עֶשֶׂר מִשְׁפָּחוֹת לְבֵית אַב אֶחָד שׁוֹמֵעַ אֲנִי שֶׂה אֶחָד לְכֻלָם, תַּלְמוּד לוֹמַר שֶׂה לַבָּיִת.