A decade ago, the 2010 Census found there were 1,561,086 Jehovah's Witnesses in Mexico (Census page > Tabular data > Basic questionnaire > Religion). And according to the recently released 2020 Census results, the number of Mexicans identifying as such has declined by 2% to 1,530,909. Both figures greatly outnumber those reported by the organization (e.g., peak publishers: 710 thousands and 874 thousands respectively), which means the former ones include children and free-riders.
For comparison, the total population has grown by 12%, the Catholic, Adventist (SDA) and Mormon (LDS) populations - by 5%, 16% and 6% respectively. Also, between 2000 and 2010 the number of JWs has grown by 36% (total - 18%, Catholic, SDA and LDS - 13%, 22% and 41%).
Some questions still remain. First, it is one of accuracy. The recent census was conducted in March 2020 so it was impacted by the pandemic: some people - more than previously - were reluctant to respond, and the verification process was significantly delayed. But it is unlikely that the undercount, if any, was significant.
Second, it's one of interpretation. It should be noted that in terms of people aged 15 and over, the JW population has grown by 7% (total - 20%, Catholic, SDA and LDS - 11%, 24% and 15%). The largest decrease was for age groups 0-4, 5-9 and 10-14 (by 1/3, 2/7 and 1/5 respectively); among those aged 15-24, it was 18%, etc. Beginning with the age group 40-44, there is a significant growth, up to between 60% and 80% for those aged 60 and over, significantly outnumbering the growth among general population. Which probably means the Jehovah's Witnesses lost many young free-riders (and perhaps active members) while still being succesful in converting older people. It is also possible that parents (especially non-JW and inactive JW ones) became more reluctant to identify their children as JW. Whatever inperpretation is correct, the recent census has some bad news for Watchtower.