Tosafos (50a, “Par He’elam) raises an interesting linguistic question.  

Why is a sin offering for the regular transgression committed by the congregation referred to as singular, ”Par” “bullock“, but throughout rabbinic literature, the sacrifice for inadvertent idolatry is referred to in the plural as, “Seiri” “ The goats of idolatry”. 

Tosafos attempts to answer but raises other questions that undermine his answer.

I did find a Ralbag that may answer this question. Ralbag (Bamidbar 15:27) on the verse that describes the sacrifice for inadvertent idolatry explains why when it comes to other sacrifices there are separate offerings for the Cohen Godol, Nasi, and an ordinary citizen, yet by the sin offering for idolatry, there is only one singular sacrifice. He explains that the message is that this sin is so great, there is no distinction between the common man and the leaders. No one is too great or too holy to not be utterly devastated and utterly contaminated by this thing and therefore they must achieve forgiveness in the identical matter with no preferences.  Zero is zero. Even Zero minus zero is still zero.  The sin of idolatry is so destructive in its aftermath, that everyone is at the zero point.

If so, perhaps the rabbis were hinting at that idea when they referred to the goat offering for idolatry as “goats“ in the plural construct. Implying that all of the congregation, from the smallest to the greatest must contend with this sin in an equal manner.