As of today I have 3 times
First was a guy passed out on my front lawn. I thought it might be a medical emergency but when the paramedics arrived it was just that he was overly drunk.
Second were two children (8-12 I guess) walking down the street in their pajamas and with backpacks. Something was not right with the picture and they were being unsafe in crossing the street.
Today. Cute dog, maybe 2 years old, sitting in the median of a busy freeway watching the traffic and panting. I would have tried to get him in my car but there is only one way that goes right and a million ways it goes wrong. So I called 911 instead.
I called in February after coming upon an elderly driver who missed the lane after a turn and, straddling the sloping median, drove her wheels right off the road. Well, one wheel, anyway, luckily she got fully stuck by the time I got there.
I’ve called a few times when I saw car accidents and reporting road hazards.
Twice that I can recall. Once because of a serious car accident that happened right in front of us. My wife ended up doing the actual talking because I jumped out of the car to help and my phone was still connected to the car.
The second time was a pocket dial from the San Diego Zoo, which I didn’t realize had happened until I got a voice mail from SDPD.
My wife’s called a couple of times, and someone once accidentally prank called them from my house, which led to an awkward conversation…
Many times. Too many to count.
Yes, I called no less than twice about a ladder in a freeway lane- different freeways, different ladders. Then I had the asshole neighbor (gone now) who was cleaning car parts with gasoline and pouring it into the gutter- in a high fire risk area. The Fire dept gave him a very stern talking to. May have been others.
Twice I can think of right now. Once was an older man who had stopped his car in traffic. He had his driver’s side door open and was sitting sideways out of it, as if to get extra room / extra air. He seemed to be having a heart attack or some other type of medical emergency. I called and stayed with him until the responders came.
The other time I was in my garage when I heard a terrific crash. I ran down my driveway to see a woman had run off the road across the street from my house, hit an electrical pole, the top of the pole had snapped off and her car was covered in live, loose electrical lines. 911 asked if she appeared to be conscious and I said I couldn’t really see from where I was and I didn’t want to get too close to the sparking lines. The first responders stood around waiting for a good 40 minutes after they showed up until the electrical folks showed up and confirmed the lines were shut down and no longer live. When they finally got to her they took her out on a stretcher and I never found out what became of her.
Car accidents, car accidents waiting to happen (ie. drunks, or possibly a medical emergency but swerving all over the road), car/wheel theft in progress, road hazards*
* depending upon your PSAP (Public Safety Access Point, the formal name for a 911 call center), calling a 10-digit phone # or 911 are answered by the exact same person & handled the exact same way, just something like a shooting gets a lot higher priority than a noise or parking complaint or a dead deer in the road.
…& as a first responder, I’ve had 911 call me! 
Yes, once when our crappy neighbors were having a screaming argument on their front lawn. It devolved into her cowering down on the driveway while he stood over her bellowing at her. There was something in the tone of his voice that made my neck hairs stand up, so we called 911 and the cops were there within minutes. They separated the two and made the guy sit on the curb while they grilled him. We overheard him say “ . . . I can be drunk in my own house!”
I’ve called several times. Once in Wyoming, a kid missed the curve by my house and rolled his car into the neighbor’s field. (He was fine.) Then I called once during lockdown because a guy was proselytizing on a street corner and leaning into car windows to pass out literature. Once I called on my way home from the gym because a woman was sitting in an odd position on the sidewalk and was unresponsive. And once on a freezing night in February when an elderly man with dementia was on his scooter in shirtsleeves, with no hat or gloves, and was trying to get to the UPS store, about a half mile away. It was pitch dark, and he had no clear idea how to get there.
Quite a few times, probably more than the average person. Quite a few car accidents have happened in front of my house as well as where I work. Plus, since I’m in a retail environment, we get the occasional person (employees and customers) that get injured. We also get a lot of people in the store that had blood work done a few minutes earlier and maybe once a year, one of them will walk in and just keel over. They’re typically fine by the time EMTs show up though. I once found an elderly man laying in the parking lot, very obviously having a stroke, but he didn’t want an ambulance, just some help back to his car (yes, I called an ambulance).
And that’s just medical issues. I’ve called, I don’t even know how many times, for police matters. People stealing, people getting argumentative to the point that they’re scaring customers. People pulling checking for unlocked cars int he parking lot. People hitting on employees so relentlessly that police intervention and being trespassed is the only way to get rid of them. Someone pointing a gun at my cashier and asking her to empty the register.
We do typically try NOT to call the police, but some people are so amped up, or high or drunk or stubborn and upset or whatever, that they’re not going to leave without being escorted by an officer. You can tell who they are because when you say “Sir, I need you to leave or I’m going to call the police” they double down instead of cutting their losses and walking away. If I tell you I’m going to call the police and you say, for example ‘go ahead, I’ll tell them to arrest YOU’, well, I’m more than happy to provide that opportunity for you.
If I had to guess, I’d say I’ve probably called 50-100 times in the last 20ish years.
Twice that I recall:
several times to report accidents. Once, because I thought my daughter was dying. (they saved her)
No, never.
Ha! I am not a first responder. But the day I realized that I live in a small town is the day one of our 911 dispatchers (a lady I went to school with) called me to tell me that the police found a disabled car in a parking lot, ran the plates, and it belonged to a guy who was living with me (a guy I went to school with) and could I please ask him to have the car towed so they don’t have to take further action. I don’t even know how she knew he was living with me, but I’m guessing Facebook.
As for the OPs question, I’ve only called once on a stranger. A lady who was clearly drunk was crossing the 5-lane road in town, not at the crosswalk, and someone needed to come scoop her up.
I called 911 when my family was at a cabin and my dad had too much to drink and he passed out and hit his head. I called from my mom’s house once when dad was sick and had passed out and hit his head again. Heads bleed a lot.
A number of times. Medical emergency, assault, fires, imminent child/elder abuse/vulnerable person abuse (I was a mandated reporter), car accidents, drunk/intoxicated drivers and/or drivers weaving, playing chicken, racing, driving in the wrong lane, enacting road rage in lethal ways.
I forgot two. 1) A fire outside my office building and 2) to report a burglary of my home.
Ah, yes. I’ve called to report a trespasser pissing on my window in the middle of the night.
I have twice. Once for a road hazard and once when my mom thought she might be having a heart attack. She was fine.
Once when I walked in on a roommate trying to take their own life. The dispatcher was insisting I stay on the phone with her while the police who showed up were asking me the same questions she was. So I handed the phone to the cop.
Once by accident, I gave my cell phone to my toddler in a stroller to watch videos while on a walk. He managed to dial 911.