If you have a lot of menu items within a particular dropdown it might make more sense to display them in 2 or 3 columns.

To do so, go to Appearance > Menus and click 'Screen Options' at the top. Tick - 'CSS Classes'

Now, open the parent menu item, whose sub items  you would like to break into columns.

Add the following to the CSS Classes field: megamenu col-2

(If you would like 3 columns, change that to megamenu col-3)