Another interpretation of "chamushim": one out of five went up. Another interpretation: one out of fifty. And some say: one out of five hundred. Rabbi Nehorai says: by the Temple service, not even one out of five hundred went up, as it is said (Ezekiel 16:7), "I made you as numerous as the growth of the field," and so forth, and it says, "And the children of Israel were fruitful and swarmed" - for they would bear six children at a single birth - and yet you say one out of five hundred went up! And when did the rest die? During the three days of darkness, as it is said (Exodus 10:23), "they did not see one another" - for they were burying their dead, and they gave thanks and praised that their enemies did not see them and rejoice in their downfall. Rabbi Yose says: they went up after five generations. (This is written at remez 564.)
How Many Israelites Left Egypt and Who Died in the Darkness
Yalkut Shimoni on Torah 227:7
דָּבָר אַחֵר, וַחֲמֻשִׁים, אֶחָד מֵחֲמִשָּׁה עָלוּ, דָּבָר אַחֵר, אֶחָד מֵחֲמִשִּׁים, וְיֵשׁ אוֹמְרִים, אֶחָד מֵחֲמֵשׁ מֵאוֹת. רַבִּי נְהוֹרַאי אוֹמֵר, הָעֲבוֹדָה, לֹא אֶחָד מֵחֲמֵשׁ מֵאוֹת עָלוּ, שֶׁנֶּאֱמַר (יחזקאל טז, ז) "רְבָבָה כְּצֶמַח הַשָּׂדֶה" וְגוֹ', וְאוֹמֵר "וּבְנֵי יִשְׂרָאֵל פָּרוּ וַיִּשְׁרְצוּ", שֶׁהָיוּ יוֹלְדוֹת שִׁשָּׁה בְּכֶרֶס אֶחָד, וְאַתְּ אַמְרֵתְ אֶחָד מֵחֲמֵשׁ מֵאוֹת עָלוּ. וְאֵימָתַי מֵתוּ, בִּשְׁלֹשֶׁת יְמֵי אֲפֵלָה, שֶׁנֶּאֱמַר (שמות י, כג) "לֹא רָאוּ אִישׁ אֶת אָחִיו" שֶׁהָיוּ קוֹבְרִין אֶת מֵתֵיהֶן וְהוֹדוּ וְשִׁבְּחוּ שֶׁלֹּא רָאוּ אוֹיְבֵיהֶם וְשָׂשׂוּ בְּמַפָּלָתָם. רַבִּי יוֹסֵי אוֹמֵר, לַחֲמִשָּׁה דּוֹרוֹת עָלוּ. (כָּתוּב בְּרֶמֶז תקס"ד).