Lots of instances also screen signups. There's no requirement for them to accept everyone. The server that Marcan (M1 Linux) and Alyssa Rosenzweig (Linux GPU drivers) are on claim a total of 700 active users for example. You'll still get new content as your users can boost (retweet) from other servers, while they act as gatekeepers by not boosting others.
Tumblr announced they would join the Fediverse. That would be interesting. Of course nothing stops Twitter from joining either.