The number shown on the invoice is the reference field and you are looking for the number field.
Try to customize your invoice with the following xpath

<!-- for the first one -->

<xpath expr="//span[@t-field='o.reference']" position="replace"> 

 <span t-field="o.number"/> 

 </xpath>

<!-- for the second one -->

<xpath expr="//p[@t-field='o.reference']" position="replace"> 

 <p class="m-0" t-field="o.number"/> 

 </xpath>

You can refer to the view report_invoice_document to locate those fields :)

have someone changing sequence after validating invoices (manually)?