After a certain point, adding more developers to a software project does not get the project done any faster. Frederick Brooks wrote about this problem in The Mythical Man-Month:
When a task cannot be partitioned because of sequential constraints, the application of more effort has no effect on the schedule. The bearing of a child takes nine months, no matter how many women are assigned. Many software tasks have this characteristic because of the sequential nature of debugging. In tasks that can be partitioned but which require communication among the subtasks, the effort of communication must be added to the amount of work to be done. […] Oversimplifying outrageously, we state Brooks's Law: Adding manpower to a late software project makes it later.
This O'Reilly Radar blog post presents the story of a CEO who did not understand this problem. The CEO hoped that adding more developers would speed up a project that was going to miss it's deadline. What did the developers do to convince the CEO that adding more developers wouldn't help, and might make things worse?
The developers each bought a copy of Brooks' book, brought the CEO into a conference room, and stacked up the copies of the book, telling him: It is extremely urgent that you read this book. We've bought you many copies so that you might read it faster. They made their point.