one only, a large celtic cross of my own design on my upper right arm.
I have 4 and I don't regret any of them. As other folks said, get something that is important or has meaning to you. That way you don't look at it and regret it.
Also, make sure you get it someplace that's easy to cover up if you are in or want to go into a professional like of work.
And lastly, research your tattoo artists. Not every shop has artists with the same talent. Look at their portfolios, ask questions (use of disposable needles is the big one), and watch them work.