Limits of Balancing Rules
Balancing out orchestral “forces” is a lifelong learning process as there are dozens of factors involved that have influence on how sections in the orchestra balance. Not only does the instrument by itself but also its number, the dynamic, the register, the style, the tempo, the players, the location, the room size and other factors play a big role in that.
The only ways to learn this is to gain experience by ideally having one’s composition played by real musicians to check how it balances or get a thorough knowledge of balances by listening to music and reading along scoresheets.
There are general rule of thumbs such as “4 woodwinds balance one heavy brass (trumpets, trombones) while 2 woodwinds balance one horn” etc. which are generally a good rule of thumb but don’t really cover up all possibilites. Even 20 flutes in their lowest register couldn’t compete with a ff trumpet while one piccolo in its highest register could easily balance out a trumpet. These things need to be looked at with much differentiation and need to be studied thoroughly.