josepp

<h3 class="order-number-label"><?php wcdn_template_title(); ?></h3>
<span class="order-date">
  <?php if( wcdn_get_template_type() == 'invoice' ) {
    echo("Nr. " . wcdn_get_order_custom_field( '_invoice_number' ));
  } ?>
</span>

This worked perfectly for me. Thank you all for the hints!

Once again, for clarity:
wcdn_get_order_custom_field( '_invoice_number' )

Edit:
I am using this together with the Sequential Invoice Numbers plugin and i think, that’s fine for germany.