Well, I've got a few thoughts on the issue.
First, our hypothetical ideas of how many crimes can be prevented by guns... well, they don't have to be that hypothetical. We can look at crime rates for various things in the US, and compare them to other countries with similar factors (such as the UK, Japan, Austrailia) and contrast the amount of gun ownership with the number of crimes committed.
Here's a study I'd like to find (though my brief searches haven't gotten anything yet): Make a list of how many violent crimes occur in each year, by country. They should be rated by as close to the same criteria as possible (I know that some countries have different definitions for "violent crimes", so hopefully that can also be taken into account somehow). Then, look at the number and rate of violent crimes that involved a firearm, violent crimes that involved a fatality from a firearm, and violent crimes that involved a fatality from something other than a firearm.
I think what you'll find is that: the US and other countries that have a lot of guns will have a lot more crimes involving guns, and a lot more fatalities that involve guns. The rate of violent crimes that don't involve guns but end in a fatality will likely be pretty close to the same between countries.
However, whether the US has more violent crimes than other countries can be... difficult. The "obvious" response is that violent crimes will be less frequent in the US, but those that are violent will escalate to a fatality much more often because guns are much easier to obtain here. Perhaps it would also be worth looking at how often a violent crime with a fatality has the aggressor as the fatality, compared to the victim; the former case wouldn't be anywhere as unfortunate as the latter. (As an aside, I'm not saying that the attacker would deserve death... but the victim wouldn't deserve the attack, either, and I'm of the opinion that if you get into a fight with someone, you can't complain if things go horribly wrong for you.)
On the other hand, I would not be surprised if the US has higher violent crime rates, despite the supposed protection of guns. The reason for this is that while the victim is much more likely to have a gun, the criminal does as well. Further, he's more likely to go into a crime prepared than a victim is, so it's not entirely clear that more people having guns will actually prevent crimes, just make the aggressor more likely to have a gun of his own before committing it.
But that applies mostly to premeditated crimes; someone lurking in a bad spot of town waiting to mug someone, or someone breaking into a store to rob it. What about spontaneous crimes, someone lashing out? Well unfortunately, guns are a hell of a lot easier to lash out with than a knife, at least if they're not kept somewhere safe and somewhat difficult to get into (and, as gun nuts would argue, what good is a gun if you can't get to it quickly in an emergency). And while a lot of people in the US are big fans of carrying around a gun, I don't think that mentality applies to carrying around a knife (even in other countries), meaning most sudden outbursts would probably end with fists instead of weapons if it weren't for concealed guns. Further, the mentality that someone has to gt into in order to pull a gun on someone else and threaten/shoot them is usually not the same mentality that would make someone stop and think that maybe that guy also has a gun, and decide to do something else.
Ok, I'm rambling here. But the point is, I want to see more data. Mainly, crime rates between the US and other countries, and not just a list of anecdotes.