Adding Multiple “Parents” in get_categories

You cannot get it in one call.

Instead, make multiple calls, then sort the data each call gave you into a single array, and take the top 5 in the new list.

e.g.

$args=array(
  'orderby' => 'count',
  'order' => 'DESC',
  'parent' => '599',
  'number' => '5'
  );
$categories=get_categories($args);

$args2=array(
  'orderby' => 'count',
  'order' => 'DESC',
  'parent' => '588',
  'number' => '5'
  );
$categories2=get_categories($args2);

// etc...

Finally you would create a new array that you will use to display the top 5 categories. You will fill this array by searching through the 5 category arrays you have just acquired, and picking out the categories with the highest count..

Think of each array as a top 10, and you’re compiling a new top 10 given 5 lists. You’d find the highest category in all the 5 lists, then the next highest, and once you’ve got 5 items you’d stop and write down your final list.