Based on estimates of legal gun ownership, which is an estimated number.
Let's take on just one issue - the sales of illegal guns in NYC. While most new guns might stay in their general locality with sales face to face, stolen guns get transported to metros for sale, and that distorts the picture.
It's been estimated thousands of guns flow into NYC and certainly thousands are confiscated from criminals there. I'd have to agree - it does sound like Pakistan with the numbers of guns being smuggled in with no effective method of preventing it.
It also makes a good pro-gun argument, same as marijuana - if they would legalize them then their would be less crime . . .