7 billion people. At a prevalence of 0.2%, that comes out at 14,000,000 globally. For 0.4% its 28,000,000. This of course presumes that prevalence is uniform and not variable due to local factors, from nutrition to medical care or cultural factors. So pick your prevalence rate and do the math.
What is perhaps misleading here is this. Many sources are worried CFS is on the increase, though they usually do not cite ME prevalence. This includes insurance companies, which comment on this from time to time. So the old prevalence rates may be meaningless today, prevalence could be much higher. Or the increase might have plateaued. We just don't know. Prevalence might even have gone down, but I seriously doubt it. The point is we don't have current data, or even very reliable data.
As
Firestormm says a biomarker or good diagnostic test is really needed before we can get reliable incidence or prevalence.
Due to the long term nature of ME its also possible that incidence is stable, but this means a steadily rising prevalence. I wish I knew the answer to this.