Above image of the first T-Shirt design I ever bought online. That number is over 50 now.
Ars Technica have a very interesting write up on the pop culture T-Shirt phenomena here
"The daily model allows you to carry no inventory whatsoever," says Matt Ingleby, CEO and one of the founding partners of RIPT Apparel. The designs that we sell, we print after they're sold... and really, for me, being in supply chain management, that was like kind of the Holy Grail of all companies. If you can run a company where you don't have to hold inventory, then you know, you're golden."