Developers, managers, or customers? I was recently involved in a situation where I felt like the customers were arbitrarily demanding for more developers on a team which already had too many developers. They were scared the project was going to be late (and it probably will be). Personally, I was scared we were going to fulfill Brook's Law.
The group of programmers already lacked in-depth business knowledge, and some were even new to the technology (.NET), yet the customer wanted to add more developers who had even less business knowledge. The impression was that this would make the project get done quicker.
I started wondering if the customer, who is extremely bright, but presumably knows little about IT project management, should really be the one determining team size.