In 1970, the number of breeding sows was about 65% larger than today. That was needed to meet demand, because there was a much lower litter rate.
To meet demand now, the hog industry is getting more piglets from sows according to the USDA Economic Research Service.
In the 70's the average litter was 7.4 piglets. On average, that was about 10 pigs produced every year per sow.
In 2016, the litter rate has bumped up to 10.6, and they farrow more often leading to an average 20.95 pigs per sow.
The USDA says the efficiency gains are because of genetic improvement to breeding stock, advancements in survival rates, and a more effective breeding and recovery period.