Sort registered users by post count? (inside admin dashboard)

Try the following code:

add_filter('manage_users_sortable_columns', 'my_user_sortable_columns');
function my_user_sortable_columns($sortable_columns) {
    $sortable_columns['posts'] = 'post_count';
    return $sortable_columns;
}

Credit to: Add additional user fields and make them sortable in the user screen