Design, Design, Design
You need only one card, but make it noticeable. Make sure it says the right thing.
I mentioned before that you can make some cards using your computer. However, I would suggest that you take time to have them designed in order to look professional. A good designer will cost at the beginning of the process but, over time, the cost will be negligible compared to the amount of business you can generate by simply using a business card. Your main card should have a simple and clear design that makes it easy for the recipients to contact you. The card should indicate what type of business you are in as well. I have seen so many business cards that leave me wondering what it is that they do. For example, I went to a fund-raising meeting for a Chamber in my local area and I was given a set of business cards for contacting possible suppliers of goods for the event. More than half the cards only had things like "Tecon Incorporated" on them. I had no idea what they did or what they could possibly supply. As a result, I only called on those companies that clearly defined what they sold.
Take a look at your own business card. Does it spell out clearly what you do? If someone picked up your business card because you left it at a potential client's office, would they be able to figure out what you do? Would they even read your card?
If you have a good design and a clear message on your business card you will be portrayed with a professional image. Placing your web site address and your email contact information will give a potential client another way to contact you. Good design should also be carried through to other components of your business stationery and website. The message you give your customers should be consistent.