According to Rodney Stark there were around 40,000 Christians in the first century in total.
See full discussion including references to Stark here:
https://infidels.org/library/modern/richard_carrier/improbable/luck.html
So technically the JW claim about 144,000 could still be correct given a few assumptions:
1. Stark is correct there were only 40,000 Christians in total in the first century.
2. The vast majority of those who professed to be Christians between apostasy in 100 AD and Bible Stidents in the late 19th century were not spirit anointed.
3. The total number of anointed in the modern period amounts to fewer than 100,000.
If the number of anointed keeps rising it will become more difficult to maintain. But as it stands it's a pretty good fit, given JW assumtptions.