1. who - refers to people
2. whom - refers to people (object form)
3. whose - refers to possession
4. which - refers to things or animals
5. that - refers to people, things, or animals
Why 5 and not 8?
It seems you might be thinking of other words that are related to relative clauses, but are not actually relative pronouns:
* what - can act like a relative pronoun in some cases, but is more commonly used as a question word
* where - a relative adverb, not a pronoun
* when - a relative adverb, not a pronoun
* why - a relative adverb, not a pronoun
Let me know if you'd like to know more about relative clauses or how to use the different pronouns!