Thank you Sebastian for quick and great answer.

You will notice that odd sections on the left side and even on right side.

I do this (the following code), but when a section is missing, it will put two section on same side. We must put counter after filtering. But how?

{{assign var="counter" value=0 }}
{{ list_sections constraints="number greater 9 number smaller 21" }}
{{ assign var="counter" value=$counter+1 }}
    {{ list_articles length="1" }}
    {{ /list_articles }}
    {{ if ! $gimme->prev_list_empty }}

<aside class="extra-articles {{if $counter is odd}}left-block{{else}}right-block{{/if}}">
<h4
class="section-title"><a href="{{ uri options="section"
}}"><b>{{ $gimme->section->name
}}</b></a></h4>
    {{ list_articles length="4" ignore_issue="false" order="bysectionorder asc" }}
    <div class="link">
        <i class="icon-double-angle-right"></i>
        <a href="{{ uri options="article" }}"> {{ $gimme->article->cim }}
                {{* <span> ({{ $gimme->issue->name }})</span> *}} </a>
    </div>
    {{ /list_articles }}
 <a href="{{ uri options="section" }}" class="solid-button pull-right">Még több</a>
</aside>
   {{ /if }}
{{ /list_sections }}


Here is the class assigned to sections. Notice the two left block one after another>

<aside class="extra-articles left-block"></aside>
<aside class="extra-articles right-block"></aside>
<aside class="extra-articles left-block"></aside>
<aside class="extra-articles left-block"></aside>
<aside class="extra-articles right-block"></aside>
<aside class="extra-articles left-block"></aside>
<aside class="extra-articles right-block"></aside>