Im not sure I agree with this. If somebody is a collector and wants to have 75 guns that should be OK, they just need to be locked up so that some random burglar doesnt have easy access to smash and grab them.
Now, a limit on ammo might not be too bad of an idea, but I dont want to touch that with a 10 foot pole.
Other than the argument for having them stolen a person can only use a max of two guns at once. With long guns you can only really use one at a time in any efficient way whatsoever unless they are full auto and you are just spray and praying.
As far as ammo, any reasonable limit you put on it won't be enough. Sure we catch people who do crazy shit with 10K rounds after the fact but how many did they actually use while doing their crazy shit? 100? 200?
I don't go to the range without at least 100 rounds and I highly doubt anyone else does either. So even "reasonable" restrictions on ammo wouldn't stop anything that has happened to date. Neither would any "reasonable" restriction on the number of guns one could own. Magazine restrictions didn't work out all that well in Cali and you can easily, and only getting easier, 3D print them anyway.