Adding category count | WordPress.org

Try this one:

function custom_add_count_on_archive_title( $title ) {
 $term = get_queried_object();
 if( $term instanceof WP_Term && 'category' === $term->taxonomy ) {
  $title .= ' ('.$term->count.')';
 }
 return $title;
}
add_filter( 'get_the_archive_title', 'custom_add_count_on_archive_title', 10, 1 );

Add this in the functions.php of your child-theme or via Code Snippet-plugin.

It works, thank you. Maybe is it possible to style the number and the parethesis by css?

Replace

$title .= ' ('.$term->count.')';

with

$title .= ' <span>('.$term->count.')</span>';

and you should be able to style it via CSS, e.g.:

.page-title span { color: red; }