He probably owns an XDm-9 and just wants to be able to buy more magazines for it.
But seriously, people arrive at a "number" purely out of theoretical assumptions. There will ALWAYS be a scenario that is outside of the theoretical parameters that the good "doctor" couldn't predict.
That's why it's so absurd to put restrictions on magazine capacities.
Lest we forget, there's already umpteen millions of high-capacity magazines already in circulation, so I guarantee you, restricting them now will have zero affect on whether they are available in the future.