while saving my jpegs, in the JPEG Options screen I accidentally save them as an 8 quality instead of the max of 12. How will this effect the larger (11x14 or 16x20) prints? I don't have the originals of all of them. If I resave them and change the quality back to 12 will that recover any lost data or is it gone for good?
No, you cannot recover data lost to image compression. Forget about that.
What's more you should know that every single time you save or re-save and close a JPEG it will get re-compressed and suffer a further loss of image quality, over and over and over again. No exceptions.
You actually use JPEGs to print 11x14" or 16x20" images? ??? !! That's just insane, but, hey it's your life.
If I resave them and change the quality back to 12 will that recover any lost data…?
Of course not. But I think you knew that.
Personally I don't use JPEGs at all, unless some clueless bloke specifically asks me for one, in which case I generate a JPEG from the final PSD or TIFF, give it to the hapless bloke and do not even keep a JPEG copy for archiving myself. If another JPEG copy is needed later on, I just generate another one from the uncompressed PSD (99.9% of all my edited files) or TIFF.