Minecraft wiki entry for Dedicated, associated text: 1 player ~ 50-100 MB of RAM - on its own, but in practicality, it will always pull more, if it's e.g. in different chunks, moving through chunks very quickly (e.g. on a boat, via rail or flying with elytra); the player object as such won't use/drain more RAM, but it will effectively lead to significantly more RAM being used by the server if it's doing anything besides standing perfectly still somewhere.