The Straw Hat Grand Fleet is a fleet formed by seven pirate crews who vowed to serve under Monkey D. Luffy and the Straw Hat Pirates.
Formed from various powerful crews who managed to succeed in the New World, and the representatives and some other members have been prime contestants of the Corrida Colosseum, the crew is very powerful indeed. In sheer numbers, they have a grand total of 5640 people(not counting the Straw Hat Pirates) from various tribes and races, being quite versatile in strength and other attributes.
Five of the seven leaders were powerful enough to defeat executives of the Donquixote Pirates.
Putting the combined known bounties of all of the “Straw Hats” together (both main crew and Grand Fleet) comes to a known total of at least 4,183,000,100 Berries.
During the Reverie arc, as the news of Luffy’s exploits in Big Mom’s territory spread across the world, the Grand Fleet was reported as well. Silhouettes of Bartolomeo and Cavendish were shown talking about it to the world. Sai and Leo then met at Mary Geoise.
The existence of the Grand Fleet was a key element in Luffy becoming the “Fifth Emperor of the Sea”.
The 5,640 Straw Hat Grand Fleet crew members may refer to Luffy’s Gomu Gomu no Mi (“5+6 = Go+mu”).