Only 13% of mothers are able to provide breast milk for 100% of their child's needs for the first 6 months, according to NPR.
http://www.npr.org/sections/health-shots/2013/09/23/22534912...
http://www.npr.org/sections/health-shots/2013/09/23/22534912...
However, that really means that you're not willing (ex a few percent who would have had dead children) to do what would have been required a few hundred years ago. Formula for a majority is a convenience... and has been since we used wet nurses and cow's milk.