If you are using Mastodon on a computer or tablet through your server's website, there's an optional Tweetdeck-style advanced web interface which lets you add lots of columns.
Here's how to activate it:
1. Log in through your server's website
2. Go to Preferences
3. Tick the box marked "Enable Advanced Web Interface"
4. Click "Save Changes" at the top
More info about the advanced interface including how to add columns and more at https://fedi.tips/how-to-activate-mastodons-tweetdeck-style-advanced-web-interface/
Worth noting that this also works rather nicely when set up as a web app on the Galaxy Fold series of phones.... it somehow automatically goes to single column when folded then the full multi-column when unfolded - it's great.
Yeah, if you resize the multicolumn mode's window to be narrower, it automatically defaults to a single column mode. Then when you enlarge the window again, it goes back to multicolumn.