How many slots does a voice server need?

A slot is one person connected to the server at the same time. Ten slots means ten people can be connected together. The eleventh gets Server is full.

Count the peak, not the members

You do not need a slot for every member of your group, only for the most people who are on at once. A clan of 80 people with 20 online in the evening needs about 20 slots, plus a little spare.

Leave some room

Add about a quarter more than your usual peak, so that a busy night, a friend dropping in or a moderator joining does not fill the server. Remember bots and your own admin login also take a slot.

What a slot does not limit

Channels. You can create as many channels as you like. Slots only count people.

Change the size later

You can move to a bigger or smaller plan from your account when your group grows or shrinks. The server keeps its channels and registered users when you change plan. See How to rename your server and change player slots for lowering the number inside your plan.

Is quality affected by slots?

No. Voice quality is set by the bandwidth limit and each user's own settings, not by how many slots you bought. See How to improve Mumble voice quality.