I'd say somewhere in the range of 10 to 30 questions is good.
As for questions I don't want to see... "You come to a crossroads. Do you go left or right?" Making people choose between left and right with no information about what happens in each direction is annoying in any case, but why people do it in personality quizzes, I'll never understand. 
Other than that, I'd say try to avoid questions with a clear "good" and "bad" answer. Generally, if people find a $10 bill on the side of the road, most people would keep it. Still, if you asked the question, "You find a $10 bill on the side of the road. Do you keep it, or try to fid the person who owns it and give it back?" a lot of people would probably say that they'd try to give it back, because they know that's the "good" answer, and they like to think they're "good" people.
Giving people a scenario asking, "How would you deal with this situation?" and giving a list of answers that demonstrate different personality traits would be a better way to go about it. I'm sure you can figure out some good questions ^_^