Unfortunately, when dealing with tasks such as software development it is nowhere near as linear as that.
The meta-tasks of each additional dev needing to be brought up to speed on the intricacies of the project, as well as lost efficiency from poor communication/​waiting on others to finish things means you usually get diminishing (or even inverse) returns from adding more people to the project. See: The Mythical Man Month
Unfortunately, when dealing with tasks such as software development it is nowhere near as linear as that.
The meta-tasks of each additional dev needing to be brought up to speed on the intricacies of the project, as well as lost efficiency from poor communication/​waiting on others to finish things means you usually get diminishing (or even inverse) returns from adding more people to the project.
See: The Mythical Man Month