It's slightly behind Final Fantasy: Brave Exvius, which I believe is Square Enix's most downloaded mobile game ever at 23 million downloads as of July. [Update: Apparently we just got a new number of 27 million, which I split off to a separate thread.]
And in case you were wondering why the game's advertising star is Alexis Ren:
Source: https://www.dualshockers.com/final-fantasy-xv-new-empire-20-million-downloads/DualShockers said:Remember the mobile title Final Fantasy XV: A New Empire by Game of War: Fire Age developer Machine Zone? Apparently it's doing very well, according to the studio's Chief Executive Officer Gabriel Leydon, as he explained in an interview on today's issue of the Japanese magazine Weekly Famitsu.
The game has passed 20,000,000 downloads worldwide, and Leydon mentions that while it's popular in Japan, North America is also a big market for the title.
The interview also included a few interesting snippets of information on the game's future. Apparently the ability to play characters other than Noctos, and Astral summoning will be implemented soon.
And in case you were wondering why the game's advertising star is Alexis Ren:
Interestingly, Leydon also talks a bit about [Alexis Ren's] involvement, explaining that she is a very talented lady, and her Instagram and Twitter accounts are tremendously popular in Japan. When fans came to the airport to welcome her, he felt that the choice was a correct one. Leydon mentions that her social media popularity was one of the reasons why she was selected, as he believes that the promotional value of an extremely popular Instagram talent is easily comparable to that of traditional celebrities.