Only a tiny minority of firearms need to be registered. My guess is that covers NFA weapons like machine-guns, which are uncommon. Virtually all typical firearms people own don't need to be registered.

No one really knows how many firearms there are in the US or who owns them. Just the fact that something like 15 million firearms are sold every year in the US gives a sense of the scale. The number of firearms in the US is staggering, no one knows the true number, and they have an indefinite lifespan if stored in halfway decent conditions.