How many rounds should you carry?

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I spend a fair bit of time on gun forums. My personal favorites are Castboolits, THR.org, and M1911. I do spend time on others (like ar15.com, etc), but most of my time revolves around those first few.  JohnKSa of THR.org put together a few graphs compiling probabilities on hit rates in a personal defense shooting scenario, with a few assumptions.

The first graph plots 'chance of success' which is defined as 4 hits on target on the Y axis with total number of rounds fired on the X axis.  Each plotted line represents hit rate. Four hits was chosen, as it could mean two hits on two different attackers. That inherently carries with it the assumption that a person transitions from one target to the next with NO error in judgement, switching immediately after the second hit is scored. This graph also assumes a constant hit probability throughout the gunfight- meaning that the first shot you fire is as likely to hit its mark as the last shot you fire. 
With those assumptions in mind, we can compute the probability of a successful hit rate to a certain threshold easily. How realistic a person's probability to maintain a steady hit likelihood throughout a gunfight is admittedly quite debatable, as injury, reload time (if applicable), cover and concealment management by the self, cover and concealment management by the attacker(s),  distance to target, and  and other unkown 'unkowns' could very realistically influence that probability throughout a fight.  However, at some point we have to accept the old adage: All models are wrong, but some are useful (G.E.P. Box, Statistician) and work with what is realistically available to us. 
Graph 2 (below) Keeps chance of success on the Y axis, but now plots hit rate probability on the X axis with each line representing the number of rounds fired (PS I fucking love logistic curves, damn near everything in life can be represented by one). I find graph 2 a bit more illuminating in its grim realities. If we consider ourselves "C students" and want a 70% chance of success (success here means 4 hits- but also means survival from a dynamic critical incident), we need to obtain a 40% hit rate probability, a full 10% higher than most LEO engagements with shots fired, and to put 11 rounds in the air while Maintaining that hit rate probability throughout the whole fight.

Think about that. For chances not much better than 2 out of 3 for coming out of a fight with 2 targets 'the winner', you need to get 11 good rounds in the air that EACH have a 40% chance of striking home. So how many rounds should you carry? It depends. The plural of anecdote isn't data, but then again Las Vegas runs on probabilities. For me, its 15 rounds. If I carry my J-frame, I carry 2 speed strips (I used to only carry 1). If I carry a 1911, it is at least 1 spare mag.

Always keep in mind though, if ever you need to defend yourself with a firearm, KNOW that if you can't get some good COVER, not concealment pretty early on in the fight, or your family will be reading a eulogy in your honor with a big emphasis on bravery.