Because so many designers are there and have an Adobe subscription, they probably have a few million. But that does not mean they are all actively uploading.
I think SS used to say they have 300k creators, istock something similar.
And if you combine that with the size of various forums and groups, I would think there is maybe 100k worldwide really doing stock seriously, with the top 10 000 probably producing the majority of the sellable content.
Now with ai Adobe might have gotten a large fresh influx, but since a lot of their production is automated, their upload volume is much higher than the number of creators.
Eyeem also claimed they had 30 million producers, but that is probably just the number of people using their app.
Although stockagencies are easy to sign up to, most people give up very, very quickly because they underestimate how much work it is and how many years it takes to get a reliable income.
eta
also you are only competing with the creators who produce for your very specific niche. For vietnamese street food, perhaps there are less than 100 creators filling that subject on a regular basis.
same for children in bed with an illness, old people needing a sleep mask...etc...once you drill down into a theme, there is much less competition than you think.
the large file numbers hide that actual detailed new content is only uploaded very little, the majority are duplicates of duplicates.