Fred Brooks wrote one of the earliest books on project management for software development: The Mythical Man Month.
And from that book comes Brooks law:
Adding more people to a project makes the project go slower
Like the ideas of slack, sustainable pace and minimising the work not done, this idea seems totally crazy to a lot of people when they first hear about it.
Built into this surprise is a mistaken assumption that the kind of work that is being done is uniform and simple (in the Cynefin framework sense). But software development is never less than complex and is sometimes chaotic. Team members need to continuously share their experiences and check understanding.
The bigger the team gets, the harder this is to do and the harder it is for any new information and understanding to reach everyone who needs to know.
There’s a point somewhere between about 7 and 12 people (the official version says between 5 & 9) when the amount of communication that needs to happen across the team to make sure that everyone knows what’s going on exceeds the value of adding another person. And beyond that, adding more people just reduces the effectiveness of the team more and more.
The universal law that says everyone must be busy all the time makes things even worse. Extra members of a team will keep themselves busy but the degree to which communication is failing is the degree to which they’re doing the wrong thing.