Exodus 12:4

ואם ימעט הבית מהיות משה — This means: if they are too few to be sufficient for one lamb — that they are unable to eat the whole of it, and it would therefore come under the category of נותר “left over”. ולקח הוא ושכנו וגו׳ THEN LET HIM AND HIS NEIGHBOUR TAKE etc. — This is what these words and those that precede it imply in its literal sense, but there is also a derivation from it of a Halachic character — that the verse intends to teach that after they have counted themselves (formed themselves into a company) for it, they may reduce their number by withdrawing from it and may register themselves for a different lamb; but if they intend to withdraw and thereby reduce their number מהיות משה — they must reduce only whilst the lamb is still in being — whilst it is alive (מחיות משה = מהיות משה) — and not after it has been slaughtered (Pesachim 89a; cf. Mekhilta). במכסת means ACCORDING TO THE NUMBER. Similar is, (Leviticus 27:23) “The number (מכסת) of thy valuation”. לפי אכלו [EVERY MAN] ACCORDING TO HIS EATING — every man who is fitted to eat the quantity prescribed by the Law; this excludes a sick person and an aged person who are unable to eat a portion the size of an olive (Mekhilta). תכסו means ye shall number yourselves, as the Targum renders it. (It is not to be translated: ye shall number others.)

and if the household be too little for a lamb, let him and his neighbour next to his house take it according to the number of the souls; according to every man’s eating shall you make your count for the lamb.

וְאִם־יִמְעַ֣ט הַבַּ֘יִת֮ מִהְי֣וֹת מִשֶּׂה֒ וְלָקַ֣ח ה֗וּא וּשְׁכֵנ֛וֹ הַקָּרֹ֥ב אֶל־בֵּית֖וֹ בְּמִכְסַ֣ת נְפָשֹׁ֑ת אִ֚ישׁ לְפִ֣י אׇכְל֔וֹ תָּכֹ֖סּוּ עַל־הַשֶּֽׂה׃