There is a common 9 round minimum requirement that is being used by those that have training. It is based on a 3 rounds for 3 attackers scenario, assuming no misses and the Mozambique drill for those 3 attackers. A 1911 with an 8-round magazine and one in the chamber meets this minimum requirement. Caliber choice has zero influence on this requirement. A secondary magazine is mandatory too, to fix malfunctions, to reload if more ammunition is needed, or to top off after the fight.
Having said that, no one ever said after a gun fight:
"Why the hell did I bring all this ammo?". There is no assurance that one will not miss and that one will not encounter more threats. Also, just because the vast majority of gun fights involve 0-3 rounds, that doesn't mean yours will. Lastly, consider this: After the gun fight is over, your gun should be in a fully loaded condition, just in case the gun fight isn't over yet. After all, you want to reload when you want to, not when you have to.
People on gun forums discuss way too much gear and way too little mindset, tactics and skill. I highly encourage people to take training classes, or at least buy training DVDs or online training from reputable instructors. I also highly encourage people to analyze past gun fights, such as by watching the
Active Self Protection YouTube channel, or by reading up on the
1970 Newhall and the
1986 Miami shootouts.