This question already has an answer here:
Suppose you've got a project that is running late. Is there any proof or argument that teams become much more productive by adding more people? I am looking for answers that can be supported by facts and references if possible.
What I'm thinking about is that existing devs have to teach the new ones (thus losing overall development time), and then the new developers have to study the code (and tasks) before they can become fully productive.