How to set products per page limit in VirtueMart

Ofcourse you may change that setting in Joomla Configuration » List Length. But also it can be done in another custom way.
To change products per page limit in VirtueMart you should open for edit file: components/com_virtuemart/virtuemart.php
Edit:

$limit = intval( $mainframe->getUserStateFromRequest( "viewlistlimit", 'limit', $mosConfig_list_limit ) );

And change $mosConfig_list_limit to any number of products per page.