The "Straw Hats" are named after Luffy's signature hat, which Luffy is the captain of ten members.

The Jolly Roger emblem is a cartoon-like depiction of traditional skull and crossbones, with the skull wearing Luffy's straw hat.

The crew currently consists of ten members, being smaller than a typical pirate crew when he first set sail, Luffy set a goal of gaining ten crewmates. Zoro, Nami, Usopp, Sanji, Chopper, Robin, Franky, Jimbei, and their captain, Luffy. 

This is in one piece the anime.