I'm looking to make my first design...how many colors am I allowed to use?

I love this shirt, but what is the color limit? I always thought it was 5. It looks great with all of them up there though!

Why is this shirt not in Navy, like the thumbnail suggests?
It would make this shirt a sure buy for me. As it stands though, no can do! Sorry, but it just needs a better color!

I FINALLY got Adobe Photoshop CS2's free trial, and now I need to know how to make a shirt. Do I scan a drawing onto the template, or what do I do? PLEASE help me because I have numerous ideas I want to submit.