If you want to get kind of “freakonomical” about this, you could come up with a number on your own.
1. Calculate the number of people who have no legs or feet, and start with that base.
2. Add to that number the estimate of newborns for, oh, the past week or so.
Those numbers shouldn’t be too hard to estimate with a fair degree of accuracy.
3. Figure the number of people worldwide who exist on less than $1 per day (you can find published estimates online for various countries, and arrive at a decent estimate), and then do a rough calculation: Of that number, probably no children under about ten years old or so would have need of shoes, so add them. Figure that probably 90% of the adult women in that population have no shoes, and add them. You can make your own estimate of the men: do some of them do something that might require shoes? But I’d say maybe 75% of the men don’t have shoes either.
There. You’ve come up with your own estimate, your own documentation, and you can refine and perfect it from now until Doomsday.