Bittybye, it sounds to me as though you have a bad one like mine. You should read this entire thread and several others about what to do about it if you cannot return it.
I don't see why it should matter how many books you have on it. Perhaps DOING THE COPYING to the Nook uses up some power, but once that is done and they are just sitting on the device, it should not matter at all, assuming you are not bouncing around from book to book constantly.
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