The plural form for the pronoun 'it' is 'they' for the subject and 'them' for the object of a sentence or clause; fro example: There is a book on my desk, it is a math book. There are books on my...
The word 'a' has no plural form; the word 'a' or 'an' specifically means one. If you want to show more than one of the noun, you use other words such a couple, a few, some, three, five, many, etc.