That's all? Seems unlikely if you only count the legal guns, and include all the fudds.
Regardless, I suspect no one turned in any mags because no one there actually owns them. Sort of like no one turned in bump stocks in MA because no one owns them. Silly NJ politicians, all this fuss over "high capacity" mags, and in the end it turns out no one actually owned any to be turned in. (Not even the thugs.)
Lot of people in NJ own those mags because prior to the recent ban(s) the limit was like 15 rounds or something like that. I just think that nobody cares that's why
nothing gets turned in. It's not as if 99% of mostly lawful gun owners in NJ are carrying their guns around on their person anyways.
-Mike