I don't have any numbers and I doubt anyone here can provide any real numbers, but I think we'll all agree that Ruger probably didn't market their guns to civilians and the real reason for replacement isn't great customer service, it's liability resulting from a defective gun with a cracked receiver injuring someone.
I'm not trying to stir the pot here, but think about this in an objective manner. The focus of Ruger's sales of the full auto models were to law enforcement since it couldn't find a military buyer that wanted their rifle. Colt was no different. They didn't offer a single M16 on the civilian market and strongly discouraged dealers from selling to civilians.
I highly doubt Bill Ruger encouraged sales of full auto guns to hobbyist collectors like us. More likely they had a large supply of AC556 receivers and chose to hold on to them after the Firearms Owners' Protection Act of May 19, 1986 which banned further production of new machine guns for civilians.
Just as Colt didn't sell directly to civilians to appease the government which was it's largest customer, I suspect that Bill Ruger's personal views coupled with his desire not to alienate sales to law enforcement agencies precluded any sales to the civilian market place.
The overwhelming majority of AC556s in private hands today have come from prisons, highway patrols, state and local police agencies where they have been retired and traded for other weapons and I'll let you speculate on which ones.
Reasoning tells me that the defective receivers are being replaced today due to an abundant supply that existed on May 19, 1986. Ruger's is wise to replace a receiver they know to be out of spec., cracked, damaged, or otherwise unfit for it's intended purpose and may cause personal injury or death. Why? It's not based on great customer service or keen fore sight, it's based on returning a know defective gun to a person and dealing with litigation afterwards if the gun KABOOMS. That's smart business on their end to avoid being sued, but you'll perceive it as great customer service.
Any guess as to how many AC556s were ever made? How many post samples are sold on a yearly basis? Who buys them these days? Just asking.