Sounds like you are on the right track. One way to tell which designs to print is with market research. Going to your potential customers to see which ones they like is one way to do that.
Another way would be to just do them ALL using a service like CafePress.com or Spreadshirt.com. You upload your designs there and they create an online storefront for you to send customers to, that way, they print them on demand, and you don't have to worry about which ones to print. They only print them when you sell a design, and you keep the commission.
Another way would be to setup a a website and offer polls to get visitors to vote on which designs are printed next.
Another way would be to just do them ALL using a service like CafePress.com or Spreadshirt.com. You upload your designs there and they create an online storefront for you to send customers to, that way, they print them on demand, and you don't have to worry about which ones to print. They only print them when you sell a design, and you keep the commission.
Another way would be to setup a a website and offer polls to get visitors to vote on which designs are printed next.