Two familiar functions of grammar in all languages are pronoun reference and plurality.
To appreciate the Rashi comment note the shifting plural and singular in verses Nu14-22:25
- And if you have erred, and not observed
all these [plural] commandments [plural], which the Lord ...
- Then it shall be, if it [singular] is committed by ignorance ...
- that all the congregation shall offer ... and they shall bring their offering, a sacrifice made by fire to the Lord, and their sin offering before the Lord, for their inadvertent sin [singular];
Thus, this grammatical point uncovers contradictory indications -
the sin is simultaneously plural and singular. Rashi resolves this contradiction using the methods of the Rashi contradiction rule.
Rashi's basic approach is that We are talking about a sin offering for one commandment that is akin to all commandments:
that is, the sin of idolatry.