Who made a pilgrimage to Jerusalem on the three Regalim?

Question

When the Temple stood in Jerusalem, and "all of Israel" were required to make a pilgrimage on the three regalim, who actually make the pilgrimage? I suppose a single city could not house all the Jews.

Answer 1

See the Mishna in Chagigah (Perek 1 - Chapter 1) :

פרק א - משנה א הַכֹּל חַיָּבִין בָּרְאִיָּה, חוּץ מֵחֵרֵשׁ, שׁוֹטֶה וְקָטָן, וְטֻמְטוּם, וְאַנְדְּרוֹגִינוֹס, וְנָשִׁים, וַעֲבָדִים שֶׁאֵינָם מְשֻׁחְרָרִים, הַחִגֵּר, וְהַסּוּמָא, וְהַחוֹלֶה, וְהַזָּקֵן, וּמִי שֶׁאֵינוֹ יָכוֹל לַעֲלוֹת בְּרַגְלָיו. אֵיזֶהוּ קָטָן, כֹּל שֶׁאֵינוֹ יָכוֹל לִרְכּוֹב עַל כְּתֵפָיו שֶׁל אָבִיו וְלַעֲלוֹת מִירוּשָׁלַיִם לְהַר הַבַּיִת, דִּבְרֵי בֵית שַׁמַּאי. וּבֵית הִלֵּל אוֹמְרִים, כֹּל שֶׁאֵינוֹ יָכוֹל לֶאֱחֹז בְּיָדוֹ שֶׁל אָבִיו וְלַעֲלוֹת מִירוּשָׁלַיִם לְהַר הַבַּיִת, שֶׁנֶּאֱמַר (שמות כג) שָׁלֹשׁ רְגָלִים

All are obligated in seeing [the Temple Mount], except for a deaf mute, an incompetent, or a minor; an intersexual, an androgyne; women, unfreed slaves; a lame person, a blind person, or a sick person; or an old person who cannot stand upon his feet. Who is a minor? Anyone who cannot ride on his father's shoulders and go up from Jerusalem to the Temple Mount, according to the House of Shammai; The House of Hillel say: Anyone who cannot hold his father's hand and go up from Jerusalem to the Temple Mount, for it says, "Three regalim" [legs] (Exodus 23:14)

In regards to the city of Jerusalem being able to house, see the Mishna in Pirkei Avos (5 - 5) which says that a miracle occurred in Jerusalem so that during the Regalim, Jerusalem was able to sustain the large number of people staying for the Chag:

עֲשָׂרָה נִסִּים נַעֲשׂוּ לַאֲבוֹתֵינוּ בְּבֵית הַמִּקְדָּשׁ {טו}. לֹא הִפִּילָה אִשָּׁה מֵרֵיחַ בְּשַׂר הַקֹּדֶשׁ {טז}, וְלֹא הִסְרִיחַ בְּשַׂר הַקֹּדֶשׁ {יז} מֵעוֹלָם, וְלֹא נִרְאָה זְבוּב בְּבֵית הַמִּטְבָּחַיִם {יח}, וְלֹא אֵרַע קֶרִי לְכֹהֵן גָּדוֹל בְּיוֹם הַכִּפּוּרִים {יט}, וְלֹא כִבּוּ גְשָׁמִים אֵשׁ שֶׁל עֲצֵי הַמַּעֲרָכָה, וְלֹא נָצְחָה הָרוּחַ אֶת עַמּוּד הֶעָשָׁן {כ}, וְלֹא נִמְצָא פְסוּל בָּעֹמֶר וּבִשְׁתֵּי הַלֶּחֶם וּבְלֶחֶם הַפָּנִים, עוֹמְדִים צְפוּפִים וּמִשְׁתַּחֲוִים רְוָחִים, וְלֹא הִזִּיק {כא} נָחָשׁ וְעַקְרָב בִּירוּשָׁלַיִם מֵעוֹלָם, וְלֹא אָמַר אָדָם לַחֲבֵרוֹ צַר לִי הַמָּקוֹם שֶׁאָלִין בִּירוּשָׁלַיִם

Ten miracles were performed for our forefathers in the Temple: (1) No woman had a miscarriage from the smell of the sanctified meat. (2) The sanctified meat never became rancid. (3) A fly was never seen in the slaughterhouse. (4) The High Priest never experienced a seminal emission on Yom Kippur [Day of Atonement]. (5) Rain never extinguished the fire of the arranged wood. (6) Wind never won out over the pillar of smoke. (7) The Omer [barley offering], the Two Loaves, and the Showbread were never found to be invalid. (8) People standing were crowded, but bowing had space. (9) Never did a snake nor scorpion ever cause harm in Jerusalem. (10) A person never said to his fellow, 'It is too crowded for me to sleep overnight in Jerusalem.

Answer 2

Pirkei Avot 5:5:

עֲשָׂרָה נִסִּים נַעֲשׂו לַאֲבוֹתֵינוּ בְּבֵית הַמִּקְדָּשׁ... וְלֹא אָמַר אָדָם לַחֲבֵרוֹ צַר לִי הַמָּקוֹם שֶׁאָלִין בִּירוּשָׁלַיִם:

Ten miracles were performed for our forefathers in the Temple: (10) A person never said to his fellow, 'It is too crowded for me to sleep overnight in Jerusalem.'

Thus, there was room to house all the Jews, theoretically.