I was on a project where we added 6 people to an 8 person team in 2 weeks. Velocity stayed exactly the same as before for a sprint or two but quality issues jum

I was on a project where we added 6 people to an 8 person team in 2 weeks. Velocity stayed exactly the same as before for a sprint or two but quality issues jumped. We then spent an enormous portion of time on bug fixing. It took about 2 months before the overall team really saw a net benefit to velocity by adding those people. Brooks exact numbers might be come from a different time, but the principle holds unless you have an uncommonly decoupled set of problems.