That number is the count of full grown cattle, 31M. When you include calves, it's 98M total in the US. It isn't easy to realize just how nauseating that truly is.

There's roughly 330M people in the US. One cow will feed 2300 people 3oz of beef, which is the most meat one may eat in a sitting and still be considered healthy (though I personally disagree, no amount of red meat is healthy IMO). 360M / 31M comes out to around 10 people per cow, which will feed them once a day 3oz for 230 days. So the US has enough beef to serve every man, woman and child a a 3oz steak serving once every 38 hours, all year long.

Of course, nothing close to this amount is actually consumed, and at least 23% of it is entirely wasted in production, 7.1M cattle annually are effectively turned into garbage before product can hit store shelves.