I’ve searched everywhere, but couldn’t figure out how to add staves to an instrument or edit the number of staves assigned to an instrument.
I’m not clear exactly what you’re asking. The number of staves varies by how many measures the piece of music is. Are you trying to create empty staves before you start entering the music? You can add more measures which will increase the number of staves an instrument will have.
I think you need to be more specific in what you’re asking before people will be able to help you. Outline what you want to get as a result and we may be able to help. Such as “I want to end up with 4 staves in my trombone part” or some such.
You can’t currently change the number of staves in an instrument, I’m afraid. This is something we plan to add in the near future.