I would guess a WordPress plugin or something.
20 million is a lot, but if you look at geoip, they are around the whole world; I took 3 random latest IPs and I saw Vietnam, Brazil and Angola. So it's not that much when it's worldwide.
But it suggests it's not a geographically limited website. If it's through a website. It's probably not a ad buy. (Who would burn money on that...)
However the requests are literally every second. So it's something very popular. (Or a bot and they are somehow faking the source address...)
> Vietnam, Brazil and Angola
Curiously, these are some of the top countries I see when analyzing traffic from malicious scraping bots that disguise themselves as old Chrome versions on my websites.
So it's possible that one of those botnet-ish residential proxy services is being used here. The ones that use things like compromised browser extensions to turn unknowing users into exit nodes.
Edit: Yep, it's residential proxies, someone on the linked page mentioned a website where you can look up the IPs and all of them come up as proxies.