Which AM4 CPU would you pick for hosting modded Minecraft servers?

For several servers at once, the Ryzen 9 5950X is the top-end AM4 option I'd consider. Its 16 cores give separate server processes more room to run alongside each other. That doesn't mean a single Minecraft server will use all those cores effectively, though.

It may be overkill if you only run a few modest servers. The 5900X gives you 12 cores, while the 5700X and 5800X are eight-core options that should still be a big step up from the 2300X.

Don't shop by clock speed alone. A newer architecture can do more work per clock, so a Ryzen 5000 chip can be much faster than the 2300X without having a dramatically higher advertised frequency.

A 5600 or 5600X is a sensible starting point if cost matters. The 5500 is another cheaper option to check. For several busy servers, though, I'd look at the eight-core models too.

I'd look at the 5700X first, then compare it with the 5800X at current prices. Both give you eight cores and a newer generation without jumping straight to the cost of a 12 or 16-core chip.

The 5800X3D and 5700X3D are worth considering if you also play games on the machine, but I wouldn't assume their extra cache makes them the best server purchase. That benefit for hosting was disputed, so compare their prices against the regular 5700X and 5800X rather than paying extra just for the X3D label.