How do selllers offer 3000-5000 stock heat tranfer designs? Do they order as they sell one of each?

Hello , I own a small shirt shop and also sell on-line. I noticed than a lot of online sellers have over 5000 stock transfer designs. I am sure they cant keep that many designs in stock. So how do they do it? Do they order as they sell one design at a time? I would like to know their trick.