Rashi (on 29:18) quotes from the Talmud Sukkah 55b that the seventy bulls symbolize the seventy nations, i.e., the seventy children of Shem, Ham and Yefet, Bereshit chapter 10.
Hizkuni (on 29:32) suggests that the number of bulls relate to the seventy special days during the year, not including Rosh Chodesh.
My guess is that the seventy bulls relate to the seventy families in the count of the people in Bemidbar 26:5-49, which relates to the seventy descendants of Yaakov who went to Egypt with him, Bereshit, 46:27, also Devarim 10:22. This connection might explain why the sacrifices are recorded soon after the count of the seventy families.
In fact, each of the three festivals has a sacrifice of the people, which connects them to the mishkan/ ohel moed/ Bet ha-Mikdash. By the festival of Matzot, the korban pesach is brought by each person prior to the festival of Matzot. This connection could be why the korban pesach is recorded in 28:15, even though there are no additional sacrifices on this day. With regard to the festival of Shavuot, 28:26 records that the day has a new minhah offering, and Vayikra 23:17 records that this minhah offering is to be brought from the people’s farms. By Sukkot, the sacrifice that relates to the people would be the seventy bulls following the idea that they represent the seventy families of the Jewish people. My son Yishai added that the main law of Sukkot is living in the booths for seven days, which is because the people lived in booths during the desert, Vayikra 23:43, and it was these seventy families who lived in these booths, so then the seventy families are recalled through the sacrifices of the seventy bulls on Sukkot.
The sacrifices by Sukkot are also unique that the number of bulls offered each day declines by one a day (from 13 bulls to 12 bulls to 11 bulls to 10 bulls to 9 bulls to 8 bulls to 7 bulls). Why is there a decrease in the number of bulls each day? If in total seventy bulls are offered why were not ten bulls offered each day? Or, why not start with seven bulls and go up to 13 bulls by the seventh day? This enters pure speculation, but here goes three reasons.
My guess is that the seventy bulls relate to the seventy families in the count of the people in Bemidbar 26:5-49, which relates to the seventy descendants of Yaakov who went to Egypt with him, Bereshit, 46:27, also Devarim 10:22. This connection might explain why the sacrifices are recorded soon after the count of the seventy families.
In fact, each of the three festivals has a sacrifice of the people, which connects them to the mishkan/ ohel moed/ Bet ha-Mikdash. By the festival of Matzot, the korban pesach is brought by each person prior to the festival of Matzot. This connection could be why the korban pesach is recorded in 28:15, even though there are no additional sacrifices on this day. With regard to the festival of Shavuot, 28:26 records that the day has a new minhah offering, and Vayikra 23:17 records that this minhah offering is to be brought from the people’s farms. By Sukkot, the sacrifice that relates to the people would be the seventy bulls following the idea that they represent the seventy families of the Jewish people. My son Yishai added that the main law of Sukkot is living in the booths for seven days, which is because the people lived in booths during the desert, Vayikra 23:43, and it was these seventy families who lived in these booths, so then the seventy families are recalled through the sacrifices of the seventy bulls on Sukkot.
The sacrifices by Sukkot are also unique that the number of bulls offered each day declines by one a day (from 13 bulls to 12 bulls to 11 bulls to 10 bulls to 9 bulls to 8 bulls to 7 bulls). Why is there a decrease in the number of bulls each day? If in total seventy bulls are offered why were not ten bulls offered each day? Or, why not start with seven bulls and go up to 13 bulls by the seventh day? This enters pure speculation, but here goes three reasons.
One, by going down on the number of bulls being offered as sacrifices instead of increasing the number of bulls each day or offering ten bulls a day means that at least for one day, the seventh day, the number of bulls match the day of the festival, seven.
Two, as suggested by the Bekhor Shor, the large number of bulls relates to the obligation to be happy on Sukkot, Vayikra 23:40 and Devarim 16:14,15, and the decline in the number of bulls being offered each day is a recognition that there is a decrease in the happiness on each successive day of the holiday. This would not be due to something intrinsically negative about the later days, but due to the difficulty in maintaining the same level of happiness over time. This would accord with the principle of diminishing marginal utility, that each additional good, here days of the holiday, provide less utility to a consumer. This would differ by the sacrifices by the holiday of Matzot which do not decline, but there is no commandment to be happy on the festival of Matzot unlike by Sukkot.
A third idea is that the countdown in the number of bulls informs us that there is something coming which is the last special day, the eighth day, Shemini Azeret, 29:35-38. In addition, maybe the countdown stops at seven bulls by the seventh day of Sukkot and then jumps to one bull by Shemini Azeret to inform us that even though Sukkot leads into Shemini Azeret still the two holidays are distinct in some ways, see our discussion in the file on Jewish holidays and customs, “Shemini Atzeret: A separate holiday from Sukkot?”