What I usually do is go through my stash and pull everything that goes with the fabrics I am coordinating with. Usually I plan well ahead of starting a project, and the idea gets refined in my mind over time. So I'll have two or three editing sessions as the idea takes shape, and I'll eliminate fabrics and add others. I think the number of fabrics is going to depend on the pattern (is there a pattern?). The pattern will give you an idea of how many fabrics you're aiming for. I use that number as the lower limit, but there's never an upper limit, if you have a fabric or scrap that seems it would work well.