Another interpretation: "according to the number of souls" (Exodus 12:4)—why is it stated? Because it says "a man," I would know only a man; from where do I know a woman, a tumtum [one whose sex is indeterminate], and an androgynos [one bearing both characteristics]? Scripture says, "according to the number of souls"—the word implies an inclusion. I might bring in these and also bring in the sick person and the minor who cannot eat an olive's worth; Scripture says, "according to his eating"—this excludes the sick person and the minor who cannot eat an olive's worth; one does not slaughter on their behalf. Rabbi Ishmael says: the verse comes to teach you that people are enrolled upon the Passover and may withdraw from it until it is slaughtered, provided that they do not leave the Passover lamb as it is [with no one enrolled]. Rabbi Yitzchak says: of what state does the matter speak? Of the living lamb.
Counting Souls and the Measure of an Olive's Worth
Yalkut Shimoni on Torah 193:2
דָּבָר אַחֵר, בְּמִכְסַת נְפָשֹׁת, לָמָּה נֶאֱמַר, לְפִי שֶׁנֶּאֱמַר אִישׁ אֵין לִי אֶלָּא אִישׁ, אִשָּׁה טוּמְטוּם וְאַנְדְּרוֹגִינוּס מִנַּיִן, תַּלְמוּד לוֹמַר בְּמִכְסַת נְפָשׁוֹת, רִבָּה מַשְׁמַע מֵבִיא אֲנִי אֶת אֵלּוּ וּמֵבִיא אֶת הַחוֹלֶה וְאֶת הַקָּטָן שֶׁאֵין יְכוֹלִין לֶאֱכֹל כְּזַיִת, תַּלְמוּד לוֹמַר לְפִי אָכְלוֹ, יָצְאוּ הַחוֹלֶה וְהַקָּטָן שֶׁאֵין יְכוֹלִין לֶאֱכֹל כְּזַיִת, אֵין שׁוֹחֲטִין עֲלֵיהֶם. רַבִּי יִשְׁמָעֵאל אוֹמֵר, בָּא הַכָּתוּב לְלַמֶּדְךָ שֶׁנִּמְנִין עַל הַפֶּסַח וּמוֹשְׁכִין יְדֵיהֶם מִמֶּנוּ עַד שֶׁיִּשָּׁחֵט, וּבִלְבַד שֶׁלֹּא יַנִּיחוּ אֶת הַפֶּסַח כְּמוֹת שֶׁהוּא רַבִּי יִצְחָק אוֹמֵר, בְּמִי הָעִנְיָן מְדַבֵּר, בְּחַי.