This is one of the most recent academic papers on JW demographics. It analyzes official membership statistics of the three churches, compares them with census data and provides estimated global figures (9-10 million Mormons, 17 million Witnesses, and 28 million Adventists - all are self-identified adherents, not necessarily active ones). I think this is a good research though some aspects of the subject remain unexplored.
This article compares the growth of three religious groups, the Church of Jesus Christ of Latter-day Saints, Seventh-day Adventists, and Jehovah’s Witnesses. All originated in America during the nineteenth century and have since globalized. It begins by using their official membership data to contrast their aggregate growth over time. It then questions the reliability of those official statistics. Noting that each group employs different criteria in selecting who it counts, it employs census data from 54 countries in all regions of the world and five surveys of US adult religious affiliation with adjustments for children as a proxy for an American census to provide a common basis for comparison. It finds consistent patterns, where membership data greatly overstate the number of Mormons, understate the number of Adventists, and also understate the number of Witnesses to an even greater extent. The article then calculates a weighted ratio between official and census data for each group and uses those ratios to estimate their aggregate adherents. This method results in a dramatic reordering of their sizes. Finally, the article accounts for the variations found between the three groups.
Click here for a PDF version of the full article: https://rdcu.be/b3Yym [a shortened link to a Springer SharedIt page]