I just checked and it looks like mstdn.social hosts ~263.2k users total with ~13k monthly users.
With the current specs (from the about page) it mostly depends on which specific CPU I got in mine and which DDR version stux/hetzner is using.
I agree that that I got isn't up to snuff and I have no clue how much CPU it takes. But I'm kinda curious how it would hold up.
My point is, 2nd hand servers can go crazy cheap.