I can' help with the number, but have done that many times by just buying the number of jewels that I think it will be, and then buying more! You will be gluing forever! My suggestion is to invest in epoxy paste, and set them into it, working very neatly and cleanly. It won't ever come loose and looks very nice. These will make lovely decorations. I'd suggest a 4 inch ball so the job doesn't last so long.
