When adding artwork to artcloud, you'll notice one of the first data fields is Edition Type, with options for Original, Limited, or Unlimited editions.
Once you've selected an Edition Type, you'll see an Editions tab appear. Here, you can input the total number of editions, each of which will be assigned its own inventory number.
To edit information about individual editions (such as price, SKU, etc.), click on the blue pencil icon to the very right of your screen.