Statistically speaking...anywhere in the rage of two or three will be all you ever need.
Statistically speaking, using simple percentage ratios of historical events to predict mutually exclusive future events is invalid.
In other words, two or three will be all you ever need, unless it isn't.
Of course, statistically speaking and using the invalid application of historical data to predict mutually exclusive future events, you don't need a gun at all. If you have a gun, you won't need to draw it. If you have to draw it, you won't need to fire it and so you don't even need to have ammo. If you do need to fire it, hitting your target isn't even absolutely critica.
...but that is if you want to trust your life to historical data that have nothing to do with you, be it for carrying a gun or how much ammo you carry.
I think it very unlikely to be confronted with a reinforced battalion of Red Chinese regulars in the daily course of commerce. The least amount I carry is for a S&W M36 - with one reload. If I haven't solved my problem in ten rounds, I'm in way over my head.
So because you are in way over your head, you give up or quit?
I cannot foresee a situation calling for suppressive fire. Then again, I consider 'suppressive fire' a well aimed and delivered shot to suppress the problem.
You may get in over your head more easily than you my realize. Waters' "The Best Defense" has a great story on how suppressive fire was used by a guy on his first trip back to a gun range after open heart surgery. It was a gun battle that took place in traffic outside of Houston where he used suppressive fire to keep bad guys from doing further harm to a trooper they had already downed.
You may find you need your suppressive fire to egress from a situation where you are in over your head.
Perhaps I'm over optimistic. Yes, I have the cell phone for back up.
If you think a cell phone is back up, you are overly optimistic. I can hear it now....
Don't come another step closer or I will call someone!