I've been a LEO firearms instructor since 1980. I have been in the unenviable position of being in two armed encounters with suspects. I survived both by the grace of God, Kevlar, and good training.
We carry a handgun because they are easier to conceal than a rifle. If we had the advantage to know exactly when a "routine" situation would turn into an armed encounter, we would carry a rifle. Since we don't have that luxury, we carry a handgun.
With any handgun, regardless of the caliber, it comes down to bullet placement. You can not make up for poor marksmanship with a larger caliber. If have told my students for decades to carry the handgun that they are comfortable with, and can shoot the most accurately. In law enforcement, that choice is sometimes made for us by agency firearms policy. In those cases you have to make the best of the situation. Train hard with what you carry. Make the time to go to the range, and also conduct dry practice, this is how you become proficient with your firearm. Just going to the mandatory qualifications every quarter, and not training between those qualifications, will not keep you at a level you really need to be at to defend your life in a gunfight. You will in fact only be keeping up with the training minimums your agency has set to hopefully keep the agency clear of a potential lawsuit.
As far as is 9 rounds enough? Well that depends on what you are comfortable with. I have carried many different types of revolvers and semi-auto pistols during my career. I have done patrol, K-9, investigations, and spent 11 years on our agencies Special Response team. Each of those assignments required a different type of handgun. They are tools, and like a mechanic, one tool does not work for everything you do.
When I carried a revolver as my primary weapon, it was always a six shot, and I carried at least two reloads for it. It may have been a 4 inch .357 Magnum, or a 2 inch Detective Special or Diamondback, Model 10 or 15, or Ruger Speed Six. I never felt I was under gunned.
With semi-autos, it pretty much the same, two reloads most of the time. During my time or SRT, I did carry three pistol magazines, because our issued thigh rig holster had a pouch on the front of the holster, so I carried and extra magazine.
At the end the day, carry what the situation dictates to you, and ultimately what makes you comfortable, none, one or twenty reloads, that's is your choice. Get good training, and remember you should never stop training.