Before I had a Janome I embroidered on another brand of machine for which I purchased a very large quantity of pre wound bobbins. They are too small for the Janome, so I just rewind them onto Janome bobbins.
Because factory wound bobbins have about 1/3 more thread on them than what we can wind, each bobbin fills about 1 1/2 Janome bobbins. I usually wind about 3/4 of the way then start a new one for the remaining thread so that I don’t have to change the bobbin so frequently.
I also recommend that if prewounds are too full that they should be rewound onto another bobbin. The Janome black bobbins are notorious for being too full and causing problems, as well as the Superior prewounds.