I think about a lot of things in terms of probabilities. When it comes to being prepared, I think one needs to weigh the probability of events and then draw a line that balances preparedness with practicality.
Personally, I think that 8+1 would cover most of the things you likely to run into, but there is always a chance, however slim, you might need more.
You may never need to draw your gun in self-defense.
You are even less likely to have to shoot.
You are even less likely to have to shoot more than 3 times.
You are even less likely than that to go through 9 rounds.
You are even less likely to go through 17 rounds.
And this goes on forever... Somewhere, you have to draw the line, lest you find yourself carrying 5 guns with 6 reloads each.
Now, as far as a magazine jam that some people brought up... again, that's a probability thing. If you have fired 2000 rounds through your carry magazine without a single failure, how likely is it that if you actually need to shoot that you hit that failure. (I am pretty much guaranteeing Murphy will pay me a visit shortly.)
I say, make sure you have a gun, make sure you know it works, and hope probability is on your side.
Anyway, my point is that you need to evaluate the situations in which you might find yourself and try to balance that with that you think is reasonable.
I think that if you think of things in those terms, you'll find your answer. Good luck.
Police always carry two extra magazines for their guns, why shouldn't you?
The same reason I don't wear body armor, carry handcuffs, or drive a car with a partition to protect me from my passengers.
Neither my job nor my personal life require me to confront people who may or may not respond with violence merely due to my presence. I do not enter a burglary scene on a regular basis. I do not respond to calls involving domestic disputes.
Most of us carry concealed. We don't wear fancy duty belts with all kinds of things hanging off of them. Keeping things light and compact is key.