This is only for public group channels where any user can join and chat as a member without an invitation. A user can also leave a group channel, in which the user can't receive messages from the channel anymore.
A user is allowed to join up to 2,000 group channels. If a user already belongs to a maximum number of group channels, the user won't be able to join another channel. For private group channels, only users who are invited as a member can join the channel.
If a user leaves a group channel, the user will no longer receive messages from the channel.