Brooks’ Law refers to a well-known software development principle coined by Fred Brooks in The Mythical Man-Month. The law, “Adding manpower to a late software project makes it later,” states that when a person is added to a project team, and the project is already late, the project time is longer, rather than shorter.
Techopedia explains Brooks' Law
Brooks' law may be applied for two key reasons:
"Ramp up" time, which is required by new project members for productivity because of the complex nature of software projects are complex. This takes existing resources (personnel) away from active development and places them in training roles.
An increase in staff drives communication overhead, including the number and variety of communication channels.