//header or woocommerce.php
cart->get_cart_contents_count(); ?>
//function file
add_filter( 'woocommerce_add_to_cart_fragments', 'iconic_cart_count_fragments', 10, 1 );

function iconic_cart_count_fragments( $fragments ) {
    
    $fragments['div.header-cart-count'] = '
' . WC()->cart->get_cart_contents_count() . '
';
    
    return $fragments;
    
}