While listening to a shiur from Rav Heinemann, he noted a Rabbi Riff - Rav in Camden and descendant of the Netziv - who was stringent for a Hagahos Ashri which says you’re supposed to have seventy aravos. Unfortunately, I was unable to find the reasoning of this Hagahos Ashri - can anyone please locate it?
The reason for taking seventy aravot on Succot was to signify the additional seventy bulls which were sacrificed on the holiday (cf. Sukka 52b); such was the custom of the Sura Academy as attested to by Natronai Gaon (Shaare Teshuvah §312). See also Otzar HaGeonim (Sukka ibid) for additional significations. The custom was ultimately codified by Tur (OC §651, end).
With regards to the Hagahot Ashri (Sukka 3:14), two things should be noted:
- He does not state that one is "supposed to have seventy", he simply quotes the Ohr Zarua (Hilchot Sukkah §308) saying "יש נוהגין" (some are accustomed to...).
- The custom there mentioned is to take seventy-one aravot, not seventy. The idea is that the numerical value of the word 'לולב' plus an added one for each of the three other species brings the total to seventy-one, signifying the number of members of the Sanhedrin (Great Assembly).