R’ Meir Arama explains the “many” to include the more prominent, less conformed Jews together with the poorer, more conformed Jews in the act of wearing sack and ash.
The Yerushalmi and Panim Acherim translate “rabim” (“many”) as meaning “the Rabbis.”
The Yosef Lekach writes that this is a reference to the reshus harabbim, the public thoroughfare. Perhaps he means to suggest that this phrase implies the participation of the general masses of Jews, like those who travel the public road. Otherwise, this may be a reference to the last stage of the five-step repentance program described in the Mishnah (Taanis 2:1), in which the aron — the synagogue Ark — is taken out into the public square, and ashes are placed on it and the community leaders.