You ask a really good question. There are reasons why you have heard many numbers because it depends a bit on production practices and your definition of a pound of beef. An old rule of thumb is that it requires 50 bushels of corn to finish an animal for our U.S. desired endpoint (USDA choice grade; USDA yield grade 2 or 3; approximately 28% body fat). There are 56 pounds of corn in a bushel, so you will need around 2,800 pounds of corn to produce an animal that weighs 1,250 to 1,350 pounds. This equates to 2.07 to 2.24 pounds of corn per pound of finished animal.
The reason you have numbers that are much higher than that may be due to many factors. Cattle convert (pounds of feed per pounds of gain) at around 5.5 to 6.5 in the feedlot. That means you need to feed about 5.5 to 6.5 pounds of diet (assuming normal finishing diet) for an animal to gain 1 pound. However, they do not enter the feedlot until they already weigh 600 to 900 pounds. During that time, they consume mostly forage prior to entering the feedlot (most producers use forage because it is cheaper and a good use of fiber that would otherwise not be harvested). So, it is misleading to say that it takes 6 pounds of corn to make 1 pound of beef. The reason you may have heard 20 pounds of corn is because not all the 1,250 to 1,350 pounds of live animal is consumed (as beef anyway). Many of the alternative meats, such as liver, intestine, etc., are consumed but not as much in the United States. We utilize the entire animal, but not all for food; an example is hides for leather. Most is harvested, but for just meat cuts, the proportion of beef in the 1,250-pound animal is much lower. For example, carcasses are generally 63 to 65% of the 1,250 pounds, or approximately 790 pounds. Some of that carcass is fat that is not consumed, and some is bone (15% or so). Therefore, edible meat cuts and ground beef may be 600 pounds. So, with the example above of 50 bushels of corn fed to a finished animal, now 4.67 pounds of corn were required for each 1 pound of beef. If you calculate the red meat yield from this and equate that to pounds of feed per pound of red meat yield, that conversion may approach as high as 20, especially using the 6 pounds for every 1 pound gain.
Clearly, it does not require 7,500 pounds of corn to get 600 pounds of beef. This would be a conversion of 12.5 to 1. Most of the beef raised in the United States, and even more so globally, consume relatively small amounts of grain. Cows that produce calves almost always utilize forage diets. Forage would not be used for human food without beef cattle grazing pasture or fed harvested forages. Lastly, many feedlots utilize other by-products from human industries. For example, in Nebraska, use of corn gluten feed and distillers grains is prevalent. These industries that produce the by-products of ethanol and high-fructose corn syrup (both human products) would be devastated if they were forced to land-fill or not feed these byproducts to livestock. In ethanol production from corn, approximately one-third of the corn grain ends up as cattle feed. This feed has no value to humans but makes an excellent cattle feed. In the scenarios above, we assumed no by-products were fed. We also used an average animal in the example above with 50 bushels of corn fed. That may vary from 1,000 pounds of corn per finished animal (with by-products and heavy cattle fed a short period of time) to as much as 3,500 pounds of corn per finished animal (with no by-products and feeding lighter calves to market weights).
The only appropriate way to answer this is to look at all the numbers. Clearly, 20 pounds is not correct because cows are usually grazing forages, not corn, and cattle do not enter the feedlot until they are 7 to 20 months of age. Clearly, 2 is not correct if you are using only red meat yield. So, the answer to the question depends how you define "beef."