Some hold more, some less, but most are 500GB.
Years ago, when I was a network admin, I was responsible for a robotic optical storage unit the size of a big refrigerator. It had 6 optical drives and a bunch of cartridges, with a mechanism to pull drives from storage and put them into the drives as well as flipping them over to use the other side. All that technology and it has the same capacity as one of these obsolete hard drives. Or half the capacity of one of these:
Something smaller than your thumbnail has twice the capacity of that beast I used to manage. All of this change in less than 20 years. Simply amazing.