Join a select form with the number of the database to check
Change your options to this:

$options = array(
      '20' =&gt; '20',
      '100' =&gt; '100',
      '500' =&gt; '500',
      '1000' =&gt; '1.000',
...
  );


Then you can use the value directly. It'll be a number. Also you need a #default_value:

#default_value =&gt; variable_get('number_select', '20')