Interesting question I've never came up with. One linguist says that he found thousands of them in his research. Wikipedia says uncountable. And while I'm writing this, some lousy nerds might be coining their new way of calling themselves to compete how weird they are :)) Anyways, I've just used three (boku / ore / zibun) personally. If you wonder what is the suitable for you, then I recommend you to find a counterpart of yourself in Japan.

To be honest, no one know's the actual number since there are a lot of different dialects. In standard Japanese, watashi, watakushi, ore, boku, atashi, etc.. I would say there are just five to 10 different variations; however, wate, uchi, atai, achiki, wai, oi, ...etc could be counted as "I" in different dialects. Since even native Japanese people cannot count all the variations, I would say it is okay for Japanese language learners to start with memorizing only ones in standard Japanese and some major dialects like Osaka or Kyoto dialects. Those are something that you learn little by little while studying this language and culture.

http://ja.wikipedia.org/wiki/%E6%97%A5%E6%9C%AC%E8%AA%9E%E3%81%AE%E4%B8%80%E4%BA%BA%E7%A7%B0%E4%BB%A3%E5%90%8D%E8%A9%9E : There are so many words for I.

They are virtually countless. Some people (mostly young girls but not always) use their own name to refer to themselves. Also it is very common that the same person uses different ways in different situations. For example, school teachers often refer to themselves as 'sensei' when speaking to their students but as 'watakushi' in front of their students' parents. They may call themselves 'otosan' when speaking to their children at home.