> I am very curious about the actual number of users of login.gov.

"Login.gov has surpassed 100 million registered user accounts. The platform facilitates over 300 million sign-ins annually and sees more than 10 million monthly active users, acting as a secure single sign-on solution across nearly 50 federal, state, and local agencies."

https://www.login.gov/partners/faq/

(It is the primary identity provider for Social Security Administration, IRS will eventually adopt it [1])

[1] IRS to adopt Login.gov as user authentication tool - https://news.ycombinator.com/item?id=30430851 - February 2022 (182 comments)