It’s an entirely hypothetical number. It could just as easily be 100. The number isn’t the point. It’s about regulating what kind of weapons are available to the public.
The government could legally say “here is a list of all guns that are considered street legal” and all others are banned/regulated. The 2nd Amendment only gives citizens the right to bear arms. It does not give them the right to an unlimited choice of what kind of weapon they want to buy. If the government said here is 1 gun anyone can legally buy and that’s it then it is technically constitutional.
Shit, it doesn’t even say firearms, just arms, but we won’t go there.