When people buy things in bulk, they expect a discount - even for domains.
If you want to maximize what you earn, I think you'll get the most selling the domains individually.
On the other hand, 21 sales versus 1 sale takes a lot of extra time. If you're busy, or can use that time to make more money, or need the cash quickly for some other project or investment, then you may be better off selling in bulk and settling for a bit less money.