That would depend on whether you're talking about transfer printing or screen printing, what equipment you already have versus what you'll need to buy and what space you have to set up in.
For instance, if you're going to be doing transfer printing using a color laser printer you already own, and you're planning to run the whole thing out of a spare bedroom, your capital needs will be fairly small. If, on the other hand, you're going to go with a fully automated screen printing press, set up in a commercial building downtown, you'll need a lot more to get started.
And don't forget to add in what you'll need to keep paying your bills and buying groceries while the business gets started. When a small business fails, it's usually because the owners were overly optimistic about how much profit their business would make and how soon it would become profitable. They didn't budget enough to pay their personal expenses for a long enough period of time.
I would suggest you contact the Small Business Administration (SBA) (http://www.sba.gov/
) They have many resources for people who are looking to start a new business that could help answer your questions.
You could also contact SCORE (http://www.score.org
), which is a non profit organization providing free business mentors. Your mentor could help you figure out your capital requirements and brainstorm ways of funding your business.
Hope this helps!