You can stack labels, but it's not the default. See How to assign more than 1 label for a file? - #2 by Leo for how to change that.
But labels are mainly intended to change the colors of files, where stacking doesn't usually make sense. (Although it can, for example if one set of labels change the color while another changes the font, or similar things that don't overlap.)
Tags may be more what you're looking for, and allow multiple per file by default. You can set those via the Metadata panel.