(In simple terms, as the number of people increases, the communications overheads also increase, generally faster, so if you have more people a greater proportion of time is wasted, hence work done doesn’t increase proportionally to the number of people. Or if you just want to inform management that more people won’t simply mean the work gets done much faster just give the example of “If takes 9 months for a woman to make a child, it doesn’t mean you can get 9 women and make a child in one month”)
The good old Mythical Man Hour.
(In simple terms, as the number of people increases, the communications overheads also increase, generally faster, so if you have more people a greater proportion of time is wasted, hence work done doesn’t increase proportionally to the number of people. Or if you just want to inform management that more people won’t simply mean the work gets done much faster just give the example of “If takes 9 months for a woman to make a child, it doesn’t mean you can get 9 women and make a child in one month”)
Diminishing Returns is the concept on a broader scale here
Ie: the more you add the less you get from adding, to the point of it becoming a complete negative