If an Ark server uses more than 12GB of RAM, we will pause it to prevent it from affecting our other customers. If this happens to you, you will see a message about it on our website. You can look at the Charts tab (under Manage Your Server) to confirm that you did exceed 12GB of RAM.
The average ARK server uses 6GB of RAM, and fewer than 1% of our customers will ever exceed 12GB. The most likely explanation for exceeding 12GB RAM is that you're using a mod map such as Crystal Isles or Skies of Nazca. These mod maps are not as well coded as the standard maps, and are more likely to leak memory.
Upgrade to a higher plan. The lowest plan (at $14/month) has a 12GB RAM cap per server, but all of the higher plans have no memory cap. It doesn't matter which higher plan you choose -- none of them have a memory cap, so they only differ in the number of slots they offer.
Switch to a different map. The standard maps are better coded, and use much less memory.