How often do they hire new forensic scientist?
I want to know how many they hire to know if I actually might have a chance to be hired in the future.
That really depends on where are you located. FBI is always hiring, but you need to be an Special Agent first. Other agencies you will probably need to keep checking. There are many forensic labs out there. Check the Society of Forensic Toxicologist website.
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.