What was the largest outlaw gang—in terms of numbers of members—in the Old West? Butch & Sundance were hardly a gang, numbering a handful at most.
Dan, of course, would know about the Wild Bunch; he’s one of the top experts on the gang.
The point you make is an interesting one. The Hollywood image of an outlaw gang—one that has a fixed number of members led by one dominant chief—really didn’t exist. Most outfits had a revolving group of participants who were used based on their availability and talents. And such gangs often had multiple leaders. That was true of the Wild Bunch, the James-Younger Gang, the Reno Brothers and others.
Some of these organizations could call on dozens of criminals or supporters to help them out in various ways, from providing supplies or hiding places to actually riding with the outlaws. But they usually didn’t take many on raids; more men meant more things could go wrong (and the loot had to be split more ways).
In terms of a single holdup, the James-Younger boys boasted as many as 14 riders (accounts differ) when they hit the bank in Liberty, Missouri, in February 1866. The take was about $60,000. Despite orders from leaders Frank James and Cole Younger (Jesse wasn’t there), one of the outlaws shot and killed a bystander. The gang cut the number of riders by nearly half after that. By 1869, when the gang robbed the bank in Gallatin, Missouri, they usually numbered about four or five. An exception was made when they went to Northfield, Minnesota, in 1876. As we know, that turned out to be a disaster. Oftentimes, smaller is better.