When one Minecraft server is not enough

Most servers start as one world and one machine, and that is the right way to start. At some point a single server stops being the right shape. Here is how to tell, and what to do about it.

Signs it is time

You want different rules in different places. PvP in an arena, peaceful building in survival, a creative world with no hunger. Mixing these on one server means a pile of per-world plugin settings.
One game mode is dragging the rest down. A minigame with fast-moving players and lots of entities can make your quiet survival world stutter.
You are afraid to restart. If updating a plugin means kicking everybody from everything, split the server.
Different people need different permissions. A builder for the creative world should not be able to touch survival.
You are over 40 or 50 players in one world. Splitting players across servers is easier on the game than a bigger machine.

A layout that works

Hub (1 to 2 GB): a small world with a spawn, portals and rules. Everyone arrives here. It almost never changes, so it almost never restarts.
Survival (4 GB and up): the main world. Add claims and rollback plugins here only.
Creative or plots (2 to 4 GB): its own world, its own permissions.
Minigames (2 to 4 GB): one server, or one per game if they are heavy.

Put a Velocity proxy in front of them so players use one address. Our network guide walks through it.

Sizing

Memory is bought by the GB, so each server only needs what it will use. A quiet hub is small. Survival and anything with big worlds or many plugins should be larger. If you are unsure, start smaller and use the memory graph on each server's Overview tab: if it stays comfortably under the limit, you have room, and if it hits the limit, grow that one server. See how much RAM a Minecraft server needs.

Sleeping helps a network

Servers that nobody is on go to sleep and cost nothing to leave around, and the first player sent there wakes them. In a network that means an event server or a seasonal world can sit ready all year, and your memory goes to the servers that are actually busy.

Keep it simple at first

Two servers behind one proxy is already a network. Add a third only when you have a reason. Every extra server is one more thing to update, back up and explain to your players.