How many of these weapons wouldn't be in circulation if the assault weapons ban hadn't been allowed to expire twenty years ago?
I don't know nor can I estimate the sum, but it's safe to say that fewer of them would be in circulation had the ban been maintained.
And, based on the evidence provided by Norway and France, would have likewise had zero effect on mass shootings. Face it, bad people can get whatever weapons they want to commit these horrible crimes. Your bans do NOTHING to prevent them.