Hello!
The "lots" feature isn't used much anymore, and I don't believe it's even an option when listing on eBay directly (however they still accept it from 3rd party tools that offer it.) I personally find it confusing for buyers, so I never have listed that way.
If you have several of the same widgets for sale and you want to allow people to buy 1 or 10 or 15 of them, the easiest thing to do is create a Fixed Price listing and enter the amount of widgets you have for sale in the quantity box. That way buyers can purchase whatever quantity they want.
If all you want to do is sell all 20 widgets together in one group to one lucky buyer, you can list it as a Fixed Price or Auction style listing, and just say in the title and description of the listing that you are selling "lot of 20 widgets."
Hope that helps! Good luck!