Currently we have Teams defaulted to show you the first (up to five) channels ever created. If you want to add more channels to your left rail view, just click the ellipses next to the channel name and select “Favorite”. When you favorite the channel, it will then be “pinned” to your left rail under that team name.
Try that and let me know if that meets your needs!
An error occurred while saving the commentSteve Gibson commented
It doesn't really make sense to only show 5. That is an arbitrary value that should be controllable by the end user.
I find it surprising that a team as large as Microsoft made a design decision to use a static value for this.