What are the requirements for a server running the new unity version with around 25 slots?

Basically the RAM requirement doesn't necessarily depend on the player count. It rather depends on how much of the world gets explored. 5 players exploring different areas of the world simultaneously will result in a higher RAM consumption than 20 players building close the spawn location

But with 25 players, it's ofc likely that not all players will hang out in the same place...

But it also depends on whether or not this is going to be a creative or survival server. If players have access to flymode etc, RAM consumption will be probably higher than on a pure survival server.

The world gen update basically increased the RAM consumption, so for 25 players, the server probably needs at least 5-10 GB, maybe even more. Unfortunately we don't have much data about the server RAM consumption yet... there was a memory leak (which was just fixed yesterday) which caused an abnormally high memory consumption on multiplayer servers.

You could start with a bit less RAM (although I wouldn't recommend running a server with less than 3 GB) and keep an eye on memory consumption. More RAM won't make the server any faster, only if RAM is full, world generation will stop (and the server will eventually crash if it runs out of memory).