I just read about such a feature request on a Facebook group. For large choral groups that feature might be interesting. I did some quick hack test implementation which then would look like this:

The problem is that if you make the main window small, you only see the lowest row:

You can even shrink the window so that it looks like only one row would be present which is not good:

The coding part is not too complicated. Basically using a QGridLayout instead of a QHBoxLayout and some other small modifications make it possible. But the question is how to configure this? Should Jamulus automatically use multiple rows if the number of faders exceeds a certain number? Or should the user configure this?

BTW: I am not planning to support this anytime soon. I just wanted to raise a feature request here to start a discussion about this feature.