Believe it or not, there have been a number of people who have done a detailed level of analysis to estimate this number.
The one I did a few years ago used the following criteria:
1. Took the total number of JW active in 1958 (1958 yearbook forward)
2. Added in the growth (baptisms) each year
3. Subtracted the death rate each year (slightly less than 1% and has been constant for a 100 years)
This gave (or should have given) the actual JWs that should have been active the following year. I left out the Disfellowshipping rate in the final analysis, because some get reinstated, and some leave for various reasons and never go back. The DF rate is about 1%, and has been steady since the mid 1950s. Also, some become "inactive" and drop out of being a JW, but are never DF'd. So, the idea is to estimate the number of former Jehovah's Witnesses (inactive or DF'd) each year compared to the number who remain active. Then total each year the study considers.
I repeated this process for each year up to 1995 (for a total of 37 years). I have not repeated the analysis since then. However, I found at that time that about 1,480,000 were DF'd, and about 800,000 were unaccounted for (inactive). That would total a little over 2.2 million by 1995.
If the rate stayed steady since 1995 (another 9 years with no change in the numbers leaving) then the number today would conservatively be about 2,730,000 ex-JWs. However, as the Internet has grown, and information has expanded, I believe that the total number of former JWs is much higher, perhaps 3.5 million to 4 million.
Maybe this is a good time for someone to do another independant study to see what the numbers look like today ... Also, I believe that the DF rate has grown from about 1% to 1.5% or so because there is evidence of an accelerated increase of JWs leaving the religion, and I expect this trend to continue until the number of former JWs equals or exceeds the number of active JWs. Just my opinion, of course.
Jim W.