I think it depends on how you define programmer. If you are just looking at people who write scrips or excel macro's etc then it might be as high as: 6billion people * 1/3 have computers * 25% of computer users write scrips = 500 million people. Or you could limit it to people who have software developer or programmer etc as part of their job title and then it's probably below 20 million.

PS: Personally, I rather prefer the most inclusive version which includes people who simply train their outlook to auto sort their mail etc. In the early days of computing there was little that separated users from programmers and I like to think we are heading back in that direction. If for no other reason than I like the idea of the maximum number of people learning how to work though problems logically.