what determines the ram usage for server?

Three primary things utilize RAM for ARK.

First and foremost, the map save file as it loads. Most of the map location data loads into RAM. For most maps this is minimal, about 1-2 GB. Some Maps have been known to add up to 3GB of RAM usage though. And if the map has VERY poor optimization, it may even have a memory leak, like Crystal Isles used to. That particular map would chew up 8GB of ram every few days.

Second, your players. When they load in, their data has to load into RAM. They take a LOT less space than the map, but they still contribute. On average, 25mb to 50mb a player. Basically every 20-40 players will use around 1GB of RAM.

Third, and finally, Mods. Ram varies as mods dynamically load portions of their data during runtime, but in general, a good rule of thumb is half the stored memory of a mod can be expected in RAM at worse case though, poorly optimized mods can use 8 times their stored space. Not going to name any names here, as most of the devs known for doing so have come leaps and bounds to get their stuff handled.