The figure that is always mentioned in the UK, ie 250,000 cfs/me sufferers always appeared a stupidly high estimate to me. Of a population of approx 63million, it means roughly 1 out of every 250 people has in the UK. Even more common than that if you look at the main demographics that it hits. Most likely those figures include anyone who just has a "fatigue" type illness, which of course means that the figures themselves are largely useless as there is no specificity to them.
People can have all the symptoms of ME/CFS, but the severity of these symptoms can vary greatly from one person to the next. One way to quantify the symptom severity of an ME/CFS patient is to determine where the patients sits on the Karnofsky scale. The Karnofsky scale is as follows:
Karnofsky Scale
100 Able to work. Normal; No complaints; No evidence of disease.
90 Able to work. Able to carry on normal activity; Minor symptoms.
80 Able to work. Normal activity with effort; Some symptoms.
70 Independent; not able to work. Cares for self; Unable to carry on normal activity.
60 Disabled; dependent. Requires occasional assistance; cares for most needs.
50 Moderately disabled; dependent. Requires considerable assistance and frequent care.
40 Severely disabled; dependent. Requires special care and assistance.
30 Severely disabled. Hospitalized, death not imminent.
20 Very sick. Active supportive treatment needed.
10 Fatal processes are rapidly progressing.
It is quite clear that the figure you settle on for the number of people with ME/CFS in any given nation is going vary depending on what level of symptom severity is required before you include them in your ME/CFS statistics.
If your statistics were to include people even with very mild ME/CFS — people at the 90 point level on the Karnofsky scale — I imagine your would have an awful lot more people classed as ME/CFS patients than if you chose say the 70 point level on the Karnofsky scale as your threshold for having ME/CFS.