Everything is spot on, exactly what you want
The service, the servers, it's nothing but fantastic. Will help you with whatever it is, even if you're slow like me they help amazingly and keep their cool. Nothing but love for the guy, an amazing soul.
Zomboid memory grows with the explored map. The game keeps every cell your group has loaded, with everything changed in it, so a six-month save needs far more than a new one, even with the same players.
Players and mods add on top, map mods most of all. Zomboid also runs on Java, which holds on to memory it has used, so a busy server sits close to its limit by design.
The calculator adds those up from data we collected on Project Zomboid servers, then leaves about 15% headroom so the server never runs right at its limit. It's a guide, not a promise, and you can upgrade from the client area at any time.
Quick answers about Project Zomboid server memory.
6 GB is enough for a small group on a fresh save. Modded community servers want 8 GB, and long-running saves with many players need 12 GB.
The game keeps every part of the map your group has explored. The longer the save runs and the more you travel, the more it holds.
A little more. The difference is small next to the explored map and your mods.
Yes. Java keeps memory it has used ready for later rather than handing it back, so high memory use on its own is not a problem.