Based on the adult game by Syrup: Many Milk.

How many anime and manga fans are there in the world?

Let’s find out. If we go to Google Keyword Planner, Google tells us 1–10 million people search for anime on a monthly basis (if you can see it). Also, Google claims for topics related to anime, around 10M to 100M (million) people search for anime EVERY MONTH. And if you search manga, it also has around 1M - 10M monthly Google searches. On Instagram there are over 58 million hashtags for the topic of “anime”. And around 25 million for “Manga” On Facebook there are multiple groups with 200 - 750,000 anime fans. An app called Amino has around 710,000+ reviews, and about 5M - 10M downloads/installs.
And more importantly, one of their anime groups has around 1M users. Now, this might not be the most helpful way to find out how many anime and manga fans are in the world. But if there are 10M - 100M people searching for anime on Google, every month.
On top of all the other numbers across social networks, we can at least say that anime & manga has well over 10M fans all over the world. But it’s probably somewhere around 50 million. And if you add how many anime fans there are offline who may not have an internet connection (yes, the whole world isn’t online yet) then it’s even more believable to be at least in the multi-millions. And let’s not forget - Manga like “One Piece” sell multi millions of copies almost on a yearly basis.
Or whenever there’s a new release.
So it’s not too far-fetched.