Adding manpower to a late software project makes it later.
– Fred Brooks in The Mythical Man Month
My own two cents is that it is better to start the project with a higher team strength and then reduce it progressively rather than adding more resources later. In addition to facilitating knowledge acquisition up- front, it helps to separate the grain from the chaff and thus retain the right team members.
The key is to identify the points when the project ramp down should happen. That is what can make or break the project.