You are right. I was off by few million, according to below stats counter. Note that any count is an estimation due to the decentralized nature of the network and instances not counted. I've seen other higher numbers too.
I think you are 10 million off, since the monthly active users shows who is actively using the platform on a monthly basis, not total registrations.
If I was to bring up Twitter's total registered users to date, it would be close to the billions. That doesn't mean there are billions of users using it monthly, this is why total registered users as a metric is irrelevant.
Twitter's DAUs are in the hundreds of millions (over 220M+). Since that is the case it is clear that the MAUs are even higher. Mastodon's 7 years of existence vs Twitter's 7 years of existence is not even close and is no contest, with Twitter at 90M+ daily active users at the time.
The growth-hacking folks around here should realize that from the perspective of the Fediverse there is no such "contest". This is probably the biggest difference why Twitter is by no means an alternative to Mastodon. There's no need to grow at all costs, move fast and break things, do crazy things to get engagement levels up, no commercial incentives, valuations, VC and shareholders to satisfy. The Fediverse is a network created by people, for people, and it is noticeable in the culture.. if you stop the frantic growth-hacking and take the time to discover it.
https://mastodont.cat/@fediverse/110191027067688818
servers: 21,583 (-17, max: 21,600) users: 8,990,211 (-69,061, max: 9,059,272) MAU: 1,469,800 (-8,596, max: 2,444,236)