I am thinking that there must be a lot and not only 15 else everyone will get all the 15 dummy questions in their exams. The computer must have been randomly picking dummy questions from the database. Statistically, it should be more than a thousand so that the chance of having two persons who knew each other getting the same question is less.
If somebody gave a tip on NCLEX questions which he took to another, and that another happened to encounter the same questions, chances are that is a dummy question.
So there is no point of really sharing exams questions to another except for good relationship.
