In short
- Count the players connected at once at peak hour, not the registered ones.
- Leave a small margin over your usual peak so nobody is left out.
- Memory depends on the game and on mods; if you only lag at peak hour, check slots and memory.
Slots: simultaneous players, not registered ones
A slot is one player connected at the same time. If your group has twenty members but rarely more than eight are on together, you need eight slots, not twenty. Check when they play: peak hour sets the number.
Leave room to grow
A community that works tends to grow. A small margin over your usual peak stops someone being left out on a Saturday night. If you fall short, you can always expand later.
Memory depends on the game and on mods
Each game asks for a different amount of memory. Open-world games with lots of buildings and entities need more, and mods or modpacks add consumption on top. Look at what the server for each game includes and do not compare across different games.
More players, more memory and more CPU
The more players, the more objects, more calculations and more traffic. A server that runs well with few players can struggle once it fills up. If you only see lag at peak hours, the problem is usually resources, not your connection.
Signs you have undersized it
The game stutters with many people, the server restarts by itself or takes a long time to save. Before blaming the game, check whether you have exceeded the slots or the memory you sized it for.