You must actually add some keys (parameters and parameter) to the videoStoryBoard converter. For example, here, we want 4 thumbnails instead of the default 9. This is the XML to be added to your Studio project (create a new XML extension in Project > Advanced Settings > XML Extensions):

<require>org.nuxeo.ecm.platform.video.convert.converters</require>
<extension target="org.nuxeo.ecm.core.convert.service.ConversionServiceImpl"
    point="converter">

  <converter name="videoStoryboard"
      class="org.nuxeo.ecm.platform.video.convert.StoryboardConverter">
    <!-- Here, we must explicitely reuse all the existing values 
         as declared in the original videoStoryboard XML converter
    -->
    <sourceMimeType>video/mpeg</sourceMimeType>
    <sourceMimeType>video/mp4</sourceMimeType>
    <sourceMimeType>video/quicktime</sourceMimeType>
    <sourceMimeType>video/ogg</sourceMimeType>
    <sourceMimeType>video/x-ms-asf</sourceMimeType>
    <sourceMimeType>video/x-msvideo</sourceMimeType>
    <sourceMimeType>video/flv</sourceMimeType>
    <destinationMimeType>image/jpeg</destinationMimeType>

    <!-- Adding this parameters/parameter tag does the thing -->
    <parameters>
      <parameter name="thumbnail_number">4</parameter>
      <!--
       <parameter name="width">100</parameter>
       <parameter name="height">62</parameter>
      -->
    </parameters>

    </converter>
</extension>