The African Blood Brotherhood peaked at only three to five thousand members

Evidence

The largest membership was in the New York home office, but there were sizable contingents in Chicago, Baltimore, Omaha and West Virginia… . The ABB also established groups in the Caribbean area; in Trinidad, Surinam, British Guiana, Santo Domingo and the Windward Islands. At its height, the ABB had only three to five thousand members, most of them ex-servicemen… . The number was kept small, in part by design, but the possibilities of danger, and the Brotherhood’s militantly nationalistic and left-wing ideology, undoubtedly alienated and confused many people. The ABB saw itself as a tight-knit, semi-clandestine, paramilitary group which hoped to act for a “worldwide federation” of black organizations. — Cedric J. Robinson, Black Marxism_ The Making of the Black…, 1016