Strategies: How many voices do I need?
Well, it depends
If you are scoring for vocals, you simply need one voice for each voice, so to speak.
If you use voices for resolving the problem of overlapping notes, for example when scoring for piano, you need two voices each time two notes overlap. If three notes overlap, you need three voices. In other words you need to check for the “worst case” (largest number of overlapping notes at a certain position) and activate that many. If you do not know how many notes you need when starting out to prepare a score, do not worry, you can add more voices later.
Voices 1 and 2 on the upper staff and 5 and 6 on the lower are special. These handle “collisions” (notes with small intervals, accidentals that otherwise would come too close, etc.) automatically which the other voices do not. Always use these voices first!
An example: in the situation below, three voices are required. The lowest note overlaps both the “melody” and the chords, so it cannot share a voice with the chords. The chords overlap the melody, so they cannot share a voice either.