If you have vector files you'd like printed, you can email them to email@example.com.
The limit is three colors, not counting the shirt's color (so effectively four colors if you're smart about it).
That's for vector printing, though. For the dollars design I had to go with raster, which is slightly inferior but still good. Those have unlimited colors, but I only accept transparent PNGs for that method, and only at resolutions of 300dpi and thousands of pixels tall or wide.