20 is definitely enough to call it defective if they are in new and cleaned condition email your seller for a return or refund with proof of the errors. im sure they will help you out.
how would you know you didn't buy one of mine
all the drives i send out get no read errors on known good test disks of each UHD type BD-66 and BD-100.
i get lots of bad drives but they are not sold im probably around 50+ defective i stopped counting a few months ago and that's out of about 300 sold so a 20-25% failure rate overall but again i dont sell those they get exchanged.