There is a classic book in software development entitled “The Mythical Man-Month” by Frederick Brooks. He debunks this idea. The issues involved fall into three categories:
There is something called the “critical path”. That is a sequence of tasks where each one depends on the previous one and none of them can effectively be divided up in a way that the pieces can be done in parallel.
More people on a project increases the communication overhead. Communication will take time from each of the people involved.
Bringing new people on the project up to speed takes time.