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!
I have two teams setup with 5 channels in one and 3 channels in the other. The one with 5 channels shows the first channel and then gives the 4 more channels drop down, the team with 3 channels displays all three, this was odd behavior, so went through looking for a setting to display all channels or increase the ones shown, couldn't find it, so went looking for an answer on the web and found that to display all the channels I need to set them all as favorites??
Just like others commenting here, I find this "automatically hiding channels that are not favorites" is just a bizarre choice, considering how expanding and collapsing works in pretty much every Microsoft application I recall using.
Please stop trying to make the UX fancy and just make the thing functional and logical, use standard expand/collapse functionality where I can choose what to have expanded/collapsed or have the channels use the available white space before hiding the extra ones that won't fit.
Even making it so that the first 10 channels PER TEAM were visible would be better than the first five channels globally, that's just poor design.