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после <a href='{$category->url()}'>{$category->_title}</a> вставить:
{{$all = \IPS\Db::i()->select( 'SUM(topics) as t, SUM(posts) as p', 'forums_forums', array( 'parent_id = ?', $category->_id ) )->first();}}
Topics: {$all['t']} Posts: {$all['p']}
по желанию:
{{$all = \IPS\Db::i()->select( 'SUM(topics) as t, SUM(posts) as p', 'forums_forums', array( 'parent_id = ?', $category->_id ) )->first();}}
{{if $all['t'] > 0}}
Topics: {$all['t']} Posts: {$all['p']}
{{else}}
{{endif}}