I wouldn't use pre-wounds for the same reasons. If I had some, I would re-wind them onto my bobbins so that I would know they were the right ones.
As for the number of bobbins one needs ... that's totally variable on the amount of sewing/quilting you will be doing, how many different threads/colours, etc. If you have thread left on a bobbin, it's important to keep it labelled or stored with the spool it belongs with.
... you think you know what it is, but after awhile, you're just not sure!
(Been There, Done That! ... and now have changed my storage habits!)