How can I make views show 10 items if there are actually less (and add placeholders)?
You can add the empty rows to the view in the View's Style output template. Assuming you are using the unformatted style, the template name would be views-view-unformatted--[view-name].tpl.php.

Using the default views-view-unformatted.tpl.php as a starting point, add the following to the top of your custom template:

&lt;?php
$num = count($rows);
$add = (10 - $num);
for ($i=0;$i&lt;$add;$i++) {
  $rows[] = '&lt;div&gt;Empty Row&lt;/div&gt;';
  $classes_array[] = 'views-row views-row-' . ($num + $i);
}
?&gt;


If you're worried about the views-row-last classes, etc, the same sort of logic could be applied earlier in the view build process using something like hook_view_pre_render(), but depending on your scenario, the above should suffice.