Minecraft has always been known as a memory intensive game, and while it is, nothing is more important to ensuring you have a lag-free and corruption free gaming experience than the amount of RAM your server is running.
The amount of memory you need is calculated with how many chunks you have active and how many players and mods are active at any given moment on the server as well as entities such as mobs, but mainly how many chunks are currently in use and what is active within those chunks. Generally, it comes down to how distanced the players are and how large the world is.
People have been known to pre-generate worlds that have a limited about of chunks to help with this.
If you are only running a small vanilla server with 3-6 people and no plugins, then you will be completely fine with 512MB-2GB of RAM over many months of building with no issues. This should facilitate 5 chunks being loaded at the same time depending on plugins. If you are wishing to play with a modpack, even with only 1 player you will need an absolute minimum of 3-4GB of RAM for the server to start properly.
If you're planning on launching a larger gaming community based on Minecraft, you'll not only be using a lot of chunks at the same time, but also many permissions and anti griefing mods and plugins, so you will need a lot more memory.
From our experience, large networks run best when using 32-64Gb of memory and in some cases more.
Over time, as players explore and build more, this causes more chunks to be active and can mean that over time, you may need to upgrade your server to a higher amount of RAM to continue to run properly.
If you have any issues with your server, or any further questions on how much RAM you would need for your server, feel free to open a ticket with our Minecraft Technical Support Team.