I don't think anyone had an accurate count on how many guns were supposed to be there.
However, the bullets used from the rifle that shot Maggie had the same ballistics as a weapon bought for Paul (who target practiced and left bullets and casings that could be used to ID the weapon). That gun disappeared, Paul lost it, according to the Murdaughs. Buster and Paul were given matching semi-automatic rifles (AR style) and Paul "lost" his. So Maggie bought him a slightly cheaper version and that is not the one that was used. No report of the theft or loss of the rifle was ever reported to police.
The shotgun was never found, and I don't think we've heard testimony about how many shotguns the family owned, but AM had another one at the house, which he says he picked up out of fear that night. So there were shotguns, plural, in the gun room. I think it was also testified that guns were kept lying about and in other areas as storage.
IMO.