That a girl and that girl? That's right? Do countable nouns have to be singular or plural in any case? If not, under what circumstances?

That a girl and that girl? That's right? Do countable nouns have to be singular or plural in any case? If not, under what circumstances?

The article is divided into definite article and indefinite article. A / an is indefinite article, not specific
The girl is a girl, so that and a can't be used at the same time