Yeppers. I personally think NICS checks is a trailing indicator of general interest in owning firearms rather than any indication about the number in people's hands. You've got all the dealer-handled transfers and such in there but--and one of you FFLs should correct me--even though we have to fill-out a 4473 for a purchase from a dealer, with a CHL/LTC I don't believe it ever hits the system as an NICS check; other states have similar license perks. If correct, there's probably some offsetting of those numbers going on. And I really thought 2021's November and December would nudge us over into a new annual NICS record total, but we didn't quite make it.
I honestly think 500 million firearms in civilian hands is quite realistic, maybe even a tad low. I mean, a decently maintained firearm will last for decades. Glock started selling handguns in 1988 (seems a lot more recent than that, but that's just because I'm getting older), and unless destroyed every single one of those should still be running; and even M1 Garands manufactured in the late '30s are still in use and changing hands. No batteries required. That's one of the very basic appeals of firearms to me: so much stuff we use in our daily lives today needs external energy to run and typically has a limited lifespan. Guns are old-school mechanical: you take care of them and they can last for generations.