The Red Hair Pirates are an infamous and powerful pirate crew ruling in the New World, captained by their chief, Red-Haired Shanks, one of the Four Emperors and a former apprentice of the Roger Pirates. They are the first pirate crew to appear in the manga and second to appear (next to the Alvida Pirates) in the anime. They are the main influences of both Luffy (he was inspired by Shanks) and Usopp’s (he wants to become a brave warrior of the sea like his father Yasopp) journeys.
In One Piece Volume 104 SBS Oda clarifies that while we often see few members accompanying Shanks, the Red Hair Pirates actually have a lot of members, it’s just that they don’t always all move together. This is why Shanks’s crew call him “Big Boss” (大頭). Oda says Shanks’s title of “Big Boss” should give an idea for the scale of his crew, which is why his top subordinates are also called the “Big Officers” (大幹部).
Oda also reveals that the Red Hair Pirates have many subordinate pirate crews under their umbrella, though he likens Shanks’s relationship with them like that of the Straw Hat Grand Fleet, where he doesn’t really “rule” over them.