It works if you can count posts by categories for an author:

function wdd_in_category_count($catslugs = ”, $display = true) {
global $wpdb;

$post_count = 0;
$slug_where = ”;
$catslugs_arr = split(‘,’, $catslugs);

foreach ($catslugs_arr as $catslugkey => $catslug) {
if ( $catslugkey > 0 ) {
$slug_where .= ‘, ‘;
}

$slug_where .= “‘” . trim($catslug) . “‘”;
}

$slug_where = “cat_terms.slug IN (” . $slug_where . “)”;

$sql = “SELECT COUNT( DISTINCT cat_posts.ID ) AS post_count ” .
“FROM ” . $wpdb->term_taxonomy . ” AS cat_term_taxonomy INNER JOIN ” . $wpdb->terms . ” AS cat_terms ON ” .
“cat_term_taxonomy.term_id = cat_terms.term_id ” .
“INNER JOIN ” . $wpdb->term_relationships . ” AS cat_term_relationships ON ” .
“cat_term_taxonomy.term_taxonomy_id = cat_term_relationships.term_taxonomy_id ” .
“INNER JOIN ” . $wpdb->posts . ” AS cat_posts ON ” .
“cat_term_relationships.object_id = cat_posts.ID ” .
“WHERE cat_posts.post_status = ‘publish’ AND ” .
“cat_posts.post_type = ‘post’ AND ” .
“cat_posts.post_author = ‘”.get_the_author_id().”‘ AND ” .
“cat_term_taxonomy.taxonomy = ‘category’ AND ” .
$slug_where;

$post_count = $wpdb->get_var($sql);

if ( $display ) {
echo $post_count;
}

return $post_count;

}