According to QuitGPT[0], as of now, 2.5 million people have done so, which they base on "website signatures, share counts on social media, and credible app usage data". I don't know how accurate this is, but seems a bit of a high number.

[0] https://quitgpt.org/

They don't claim that's 2.5M cancellations of paid subscriptions. It's "taken action". It's vague for a reason