Found an actual study.

Data was collected from 203 participants at the management/executive level in seven companies. Paul Babiak does consulting work and was able to get the cooperation of the organizations involved. He was able to carry out actual interviews and had access to personnel files and various performance appraisals. In addition he was able to observe the individuals at work and interview their coworkers. Based on this information he completed the Psychopathy Checklist-Revised on each participant (consulting with Robert Hare where necessary). It was discovered was that nine participants had a score of 25 or more, which is associated with psychopathy. This comes out to about 1 in 25.

Note: the average scores on the PCL-R were lower than those found in the general public, but the number of psychopaths was higher.

The paper points out the difficulties involved in getting the necessary cooperation to carry out a large-scale study. A sample size of 203 is rather small to get an accurate result.