If we keep metrics of how our Project Velocity changes as we add developers to the Team we can begin to get a rough estimate for how it will affect the Project Burndown.
In this blog post Mike Cohn estimates that adding a seventh developer to a six man team will reduce the velocity by approximately the amount you’d have expected it to rise by in the next iteration. In the second iteration following you will still be below your original velocity and it’s not until the third iteration following that you’ll gain your expected increase in velocity. Note also that the total gain in velocity will be less than ⅙th.
Any developer or project manager instinctively knows this and the reasons have been described clearly. For instance, there are four stages a team goes through: forming; storming; norming and performing  (Bruce Tuckman (1965)). More forcefully, there’s Brook’s Law: “Adding manpower to a late software project makes it later.” (or “Nine women can’t make a baby in one month.”).