管理画面で他のユーザの投稿数やアップロード数を表示したくない

以下のコードをfunctions.phpに記述してみてください。

// Show only posts and media related to logged in author
add_action('pre_get_posts', 'query_set_only_author');
function query_set_only_author($wp_query) {
	global $current_user;
	if(is_admin() && !current_user_can('edit_others_posts')) {
		$wp_query->set('author', $current_user->ID);
		$screen = get_current_screen();
		add_filter('views_'.$screen->id, 'fix_post_counts');
		add_filter('views_upload', 'fix_media_counts');
	}
}

// Fix post counts
function fix_post_counts($views) {
	global $current_user, $wp_query;
	unset($views['mine']);
	$types = array(
		array('status' =>  NULL),
		array('status' => 'publish'),
		array('status' => 'draft'),
		array('status' => 'pending'),
		array('status' => 'trash')
	);
	foreach($types as $type) {
		$query = array(
			'author'      => $current_user->ID,
			'post_type'   => 'post',
			'post_status' => $type['status']
		);
		$result = new WP_Query($query);
		if($type['status'] == NULL):
			$class = ($wp_query->query_vars['post_status'] == NULL)  ? ' class="current"' : '';
			$views['all'] = sprintf(__('<a href="%s"'. $class  .'>' . __('All') . ' <span class="count">(%d)</span></a>', 'all'),
				admin_url('edit.php?post_type=post'),
				$result->found_posts);
		elseif($type['status'] == 'publish'):
			$class = ($wp_query->query_vars['post_status'] == 'publish') ? ' class="current"' : '';
			$views['publish'] = sprintf(__('<a href="%s"'. $class .'>' . __('Published') . ' <span class="count">(%d)</span></a>', 'publish'),
				admin_url('edit.php? post_status=publish&post_type=post'),
				$result->found_posts);
		elseif($type['status'] == 'draft'):
			$class = ($wp_query->query_vars['post_status'] == 'draft') ? ' class="current"' : '';
			$views['draft'] = sprintf(__('<a href="%s"'. $class .'>'. __('Draft') . ((sizeof($result->posts) > 1) ? "s" : "") .' <span class="count">(%d)</span></a>', 'draft'),
				admin_url('edit.php?post_status=draft&post_type=post'),
				$result->found_posts);
		elseif($type['status'] == 'pending'):
			$class = ($wp_query->query_vars['post_status'] == 'pending') ? ' class="current"' : '';
			$views['pending'] = sprintf(__('<a href="%s"'. $class .'>'. __('Pending') .' <span class="count">(%d)</span></a>', 'pending'),
				admin_url('edit.php?post_status=pending&post_type=post'),
				$result->found_posts);
		elseif($type['status'] == 'trash'):
			$class = ($wp_query->query_vars['post_status'] == 'trash') ? ' class="current"' : '';
			$views['trash'] = sprintf(__('<a href="%s"'. $class .'>'. __('Trash') .' <span class="count">(%d)</span></a>', 'trash'),
				admin_url('edit.php?post_status=trash&post_type=post'),
				$result->found_posts);
		endif;
	}
	return $views;
}

// Fix media counts
function fix_media_counts($views) {
	global $wpdb, $current_user, $post_mime_types,  $avail_post_mime_types;
	$views = array();
	$count = $wpdb->get_results("
		SELECT post_mime_type, COUNT( * ) AS num_posts
		FROM $wpdb->posts
		WHERE post_type = 'attachment'
		AND post_author = $current_user->ID
		AND post_status != 'trash'
		GROUP BY post_mime_type
	", ARRAY_A );
	foreach($count as $row)
		if ($count && $row != 0) {
			$_num_posts[$row['post_mime_type']] = $row['num_posts'];
			$_total_posts = array_sum($_num_posts);
			$detached = isset($_REQUEST['detached']) || isset($_REQUEST['find_detached']);
		};
	if (!isset($total_orphans))
		$total_orphans = $wpdb->get_var("
			SELECT COUNT( * )
			FROM $wpdb->posts
			WHERE post_type = 'attachment'
			AND post_author = $current_user->ID
			AND post_status != 'trash'
			AND post_parent < 1
		");
	$matches = wp_match_mime_types(array_keys($post_mime_types), array_keys($_num_posts));
	foreach ($matches as $type => $reals)
		foreach ($reals as $real)
			$num_posts[$type] = ( isset($num_posts[$type])) ? $num_posts[$type] + $_num_posts[$real] : $_num_posts[$real];
	$class = (empty($_GET['post_mime_type']) && !$detached && !isset($_GET['status'])) ? ' class="current"' : '';
	$views['all'] = "<a href='upload.php'$class>" . sprintf(__(__('All') .' <span class="count">(%s)</span>', 'uploaded files'), number_format_i18n($_total_posts)) . '</a>';
	foreach ( $post_mime_types as $mime_type => $label ) {
		$class = '';
		if (!wp_match_mime_types($mime_type, $avail_post_mime_types))
			continue;
		if (!empty($_GET['post_mime_type']) && wp_match_mime_types($mime_type, $_GET['post_mime_type']))
			$class = ' class="current"';
		if (!empty( $num_posts[$mime_type]))
			$views[$mime_type] = "<a href='upload.php?post_mime_type=$mime_type'$class>" . sprintf(translate_nooped_plural($label[2], $num_posts[$mime_type]), $num_posts[$mime_type]) . '</a>';
	}
	$views['detached'] = '<a href="upload.php?detached=1"' . ($detached ? ' class="current"' : '') . '>' . sprintf(_x('Unattached <span class="count">(%s)</span>', 'detached files'), $total_orphans) . '</a>';
	return $views;
}