> How many users does Matrix have?
It's difficult to say, as the French government [0], German armed forces [1], and several thousand independent users/corporations/groups run their own servers. What we can determine is that matrix.org on its own has >10 million accounts [2] (search for "global visible accounts" in the link).
> Hosting an own server is not possible on a small root server you can get for 5 EUR/USD a month, it requires signifcant [sic] resources
A direct example of setting up Matrix is shown here [3], using a $20USD/month Digital Ocean droplet. It's a reasonable approximation, though inexact, as we don't know how many users this server will be supporting. This example is thrown further off, as it's the Matrix project lead setting up the server. He's likely in many rooms with high user counts and, as high user counts dramatically increase resource needs, likely needs more RAM and CPU than someone setting up a server for a group of friends.
I think it's gotten much easier to stand up a matrix-synapse server than when you last tried, and suspect you left with a bad taste after joining popular rooms that (due to how many users they have) lag much more than you'd typically experience. Perhaps I'm wrong and you did everything right just last week and still don't like it, but I'd encourage more people to see for themselves :)
[0] https://matrix.org/blog/2018/04/26/matrix-and-riot-confirmed...
[1] https://element.io/blog/bwmessenger-goes-live-for-bundeswehr...
[2] https://matrix.org/faq/
[3] https://www.youtube.com/watch?v=dDddKmdLEdg