According to the Bureau of Labor statistics, there were about 15,400 jobs for forensic scientists in the US in 2016 and that number will likely grow in the future: https://www.bls.gov/ooh/life-physical-and-social-science/forensic-science-technicians.htm.
You are asking "how often" people get hired and that is a different question, though, because that would require you to know how many people apply for each job. The answer to this question would really depend on where you live and what kind of background you have that makes you a good candidate. However, you should be aware that "employment of forensic science technicians is projected to grow 17 percent from 2016 to 2026, much faster than the average for all occupations." so it looks like forensic scientists would be in demand and will get hired more often than before.