Person for one and people for two or more. However 'persons' is seen in legal documents and literature, it is formal English. Using 'persons' in conversational English sounds strange and should be avoided.

Exactly. We just say "people" instead of saying "persons"

I've read we can use persons for exact number of people, but I really don't know is it true or not? If yes or no, is there any reference?

I've read we can use persons for exact number of people, but I really don't know is it true or not? If yes or no, is there any reference?

"People" is the plural of "person".