If you find you’re creating too many channels by replying to so many emails, you can change your view settings to only see the important ones. Follow Slack’s guide on organizing your channels.
Extra tip: press ⌘+K on Mac—Ctrl+K on Windows/Linux to navigate between your channels.
To more directly reduce your number of channels (instead of simply hiding them) you can use the @mailclark archive command to archive all channels which have not been used in the past 7 days.
If you want to customize the number of days, simply type it at the end of the command—e.g. @mailclark archive 30 will archive all channels which have been inactive for the past 30 days.
Click your team name in the top-left corner → Preferences → Advanced Options