Twelve to 20 experience based questions is about all you can ask in an interview that lasts from 45 minutes to 1 1/2 hours. However the number of questions you ask is based on the number of distinct skills that you are looking for. For each interviewer on your team, you should prepare at least two questions per skill that the interviewer is responsible for assessing. For example, if my team was made up of three interviewers and we were all gathering data about the same six skills, I would need to develop 36 questions (3 interviewers x 6 skills x 2 questions/skill).
Also, Don't give any two interviewers the same questions to ask. The idea is to get as much data from a candidate as you can without giving the candidate and opportunity to rehearse. If more than one interviewer asks the same question, you can see how this might compromise the quality of the data you get.
