I don't think is millions. Take into account that a ddos attacker is not going to execute JavaScript code, at least not any competent one, so they are not going to run the PoW.
In fact the unsolicited and unwarned PoW does not provide more protection than a captcha again ddos.
The mitigation comes from the smaller and easier requests response by the server, so the number of requests to saturate the service must increase. How much? Depending how demanding the "real" website would be in comparison. I doubt the answer is millions. And they would achieve the exact same result with a captcha without running literal malware on the clients.
Not really. I only ask because people always say it's for LLM training, which seem a little illogical to me, knowing the small number of companies that have access to the computer power to actually do a training with that data. And big companies are not going to scrape hundreds of times the same resource for a piece of information they already have.
But I think people should be more critique trying to understand who is making the request and with which purpose. So then people could make a better informed decision of they need that system (which is very intrusive for the clients) or not.