Seems like tons of Mastodon advocates are the ones always pitching it as an alternative to Twitter. Every time that happens, I'll just get the numbers out since you brought up the total number or registered users which is irrelevant to active users:

Twitter: 220M+ DAU over 17 years.

Mastodon: ~50,000 DAU (1.4M MAU) over 7 years.

What matters in social networks is scale and it still means more users and there is already evidence that Mastodon not only was and is still struggling to scale, but eventually needed to re-centralize to handle their scalability issues, ie. using Cloudflare. Most of the instances there, are hosted by enthusiasts and hobbyists which intermittently struggle with more users.

If an instance is too small, it will easily be at risk of getting attacked and overloaded with users, with the instance falling over and having to close signups. If its too large, they become more centralized and to incentivise federation they have to close signups. Either way, centralization is inevitable.

It's already bad enough for normal users to 'choose an instance' if their can't register on a closed one or if a rouge admin bans you or an entire instance over something silly. Thus, It is quite clear to me, that for normal users who are not techies or geeks, Mastodon is not a viable alternative or even remotely to replace Twitter or even a recommendation.