Exodus 12:30

ויקם פרעה AND PHARAOH AROSE from his bed, לילה AT NIGHT and not, as is the way of kings, at three hours in the day (9 o’clock in the morning) (Mekhilta; cf. Berakhot 9b). הוא HE, first — and afterwards עבדיו HIS SERVANTS — this informs us that he went round to the houses of his servants and made them get up (Mekhilta). כי אין בית אשר אין שם מת FOR THERE WAS NOT A HOUSE WHERE THERE WAS NOT ONE DEAD — If there was a firstborn there, he died: if there was no firstborn there, then the chief person in the house died because the chief person in the house is called בכור, as it is said, (Psalms 89:28) “I also will appoint him to be a בכור, [the highest of the kings of the earth]” (Mekhilta DeRashbi 12:30; cf. Rashi on 4:22; Pesachim Rabbah, Midr. Vajosha). Another explanation is: the Egyptian women were unfaithful to their husbands and bore children from young men, unmarried, and thus they (the Egyptian men) had many firstborn sons sometimes there were five to one woman, each being the firstborn to his own father (thus there might well be no house in which there was not a firstborn) (cf. Mekhilta).

וַיָּ֨קׇם פַּרְעֹ֜ה לַ֗יְלָה ה֤וּא וְכׇל־עֲבָדָיו֙ וְכׇל־מִצְרַ֔יִם וַתְּהִ֛י צְעָקָ֥ה גְדֹלָ֖ה בְּמִצְרָ֑יִם כִּֽי־אֵ֣ין בַּ֔יִת אֲשֶׁ֥ר אֵֽין־שָׁ֖ם מֵֽת׃

And Par῾o rose up in the night, he, and all his servants, and all Miżrayim; and there was a great cry in Miżrayim; for there was not a house where there was not one dead.