I have been wondering how to determine the optimal balance of voices in SATB music. I have found quite a bit of material on how to arrange the parts but less on the numbers. The arrangement issues were interesting and overlapped my previous research on the positions of the strings in an orchestra. However, I found one layout suggestion that I have never seen or heard of for strings: the voices are mixed: not all the sopranos were together.
The question is quite broad so I will try to narrow it a bit.
Assume that all of the singers are of a similar ability. So, we are not dealing with the challenge of mixed abilities. At least good amateurs.
I am only interested in classical genres though answers on other genres are welcome.
Some specific examples.
Pure choral music e.g. Christmas carols. Moderate forces such as a dozen or two.
Large scale pieces with orchestra e.g. Beethoven's 9th or Verdi's Requiem.
So, how to choose how many sopranos, altos, etc?