Stated by Fred Brooks in his 1975 book The Mythical Man-Month. It is described well by John Drummond:
"Brooke's Law states that programming work performed increases with direct proportion to the number of programmers (N), but the complexity of a project increases by the square of the number of programmers (N2). Therefore, it should follow that thousands of programmers working on a single project should become mired in a nightmare of human communication and version control."
See also The Cathedral and the Bazaar