how many diffent arts are there in the whole world

First of all, the subject is not "3rd grade" but "ART" or "ARTS" and this is such a broad question!

The arts encompasses visual arts, literature and the performing arts - music, drama, dance and film, among others. There are martial arts, and all kinds of that. There are also design arts.

This should get you started. Look around and see what else you can think of.

Sra