That is a qustion that is yet to be solved. By my Experience I think you need 5 in each section but everybody has to play with the same air speed and in tune or else you would have a weird sound and a section that is over blowing and you don't have a balanced sound.

Its a proportion thing first of all, dependent on how many people you have - normally the more instruments you have the easier it is to blend. For the brass section, its important to have a solid bass; however, since low brass is naturally quite loud, you don't need as many; in this way it kinda balances out. Let's assume there's 30 people in the brass section. I'd recommend: 6 Tubas 9 Trombones/Baritones 6 mellophones 9 trumpets The key is to have a full sound in each section, and at the same time be able to blend with the other sections. The tubas and trombones must support each other and actually be fuller/louder than the high brass. High brass needs to be balanced with mellos otherwise it doesn't have that nice brass ring to it and the trumpets stick out like a sore thumb. I'm only a senior in high school, but thats my two cents on the subject. Its a tricky business though and a lot depends on how the musicians play. By the way, for the woodwinds, it doesn't really matter as much for balance; because its a struggle to hear them from the stands anyway. A good rule of thumb for woodwinds is get as many as possible (or make them switch to brass). Flutes and saxes are easier to hear than clarinets, so usually more of the clarinets is prefered.