Good point.
That would put OpenAI around #5 on this list (by estimated subscriber count): https://largest.org/technology/largest-saas-businesses-by-number-of-subscribers/
Less than Microsoft, Google, SalesForce and Zoom - but higher than Slack, DropBox, and Adobe Creative Cloud.
It's surprising and rare for a relatively new company to jump that high in user base this quick.
It's surprising, but it's plausible. OpenAI and derived products do anecdotally seem about that popular, this year.