I would like to count the amount of assets to do a simple if else in the template

If more then one asset, display this html code else display some other html code I can't get it to work. Tried everything I could possibly try...Searched the docs, nothing worked... o.O

here I need to check if assets:header_image has more then 1 image
...
{{ if header_image }}
  <div class="header_image">
    {{ assets:header_image }}
    <div class="header_image-image">
    // if it has one image
 ... show this html code
    // else
//    show this html code
    </div>
    {{ /assets:header_image }}
  </div>
{{ /if }}
...

Part of the page content:

header_image:
  - slideshow/startseite/slider_startseite_2048x650_1.jpg
  - slideshow/startseite/slider_startseite_2048x650_3.jpg
  - slideshow/startseite/slider_startseite_2048x650_2.jpg