The plural sheep are (na) caoirich. It’s English that is weird in that it has the singular = plural in this word, in Gaelic those are two different forms.
A’ chaora is singular, the one sheep. You never get lenition after the article in plural.
(Also you probably just mixed the terms, but nach robh definitely has tense information – it’s the past tense. But it does not inflect by number, here it’s the noun that gives number.)