It’s all over the spectrum and varies greatly from one state to another. My state, Tennessee, it’s about a four way tie right now between 9, 40, 357sig, and 45. Ten years ago almost no one was issuing 9mm except a few feds. Its not as accurate as it once was but I have a list for TN found here: LE Duty Handguns in the State of TN - Handguns - TNGunOwners.com

With the exception of Atlanta PD, most large agencies do not change handguns very frequently. In fact, smaller agencies get newer gun about three times as often as much larger agencies. I worked for a department with a small budget and only six full-time. They have changed handguns twice since I left in 2012. A while back, some department, I think St. Louis or Springfield, MO, got new handguns for the first time in 20 years.