when I had racks of thread I had a lot of bobbins filled with the full spectrum of colors.
I've been using up thread for the last three years and am down to just a few spools.
Last week I bought 3 big spools of 50 weight neutrals and will use that to fill bobbins now.
.
Yes I prefill, I have a thin drawer in what what once a tool box, when it gets down to 3 or 4
I fill another 10 bobbins or so. Such a simple and easy job for what is an essential to our craft!