As far as I understand it, the article determines whether it's plural or singular. Correct?
Nā for Plural
Ke and Ka for singular
This is the definite article "the", and there is only nā to express the in the plural form.
There are other ways to express plural when not saying the definite article "the", such as some, these, those, possessive articles like my or his etc which would be plural. In those cases, a different word is used - mau. It will be taught eventually.