Hello Duvan, 

By default, in Odoo Version 12 when you will cancel an invoice then you cannot see the invoice number. 

To unhide the invoice number in Odoo V12 you need Inherit invoice_form; code to achieve this is

<record id="invoice_form_draft_inherit" model="ir.ui.view">

            <field name="name">invoice.form.draft.inherit</field>

            <field name="model">account.invoice</field>

            <field name="inherit_id" ref="account.invoice_form"/>

            <field name="arch" type="xml">

             <field name='number' position="replace">

              <field name="number" readonly="1" class="oe_inline"/>

              <field>

            </field>

        </record>

Hope your query is resolved by now. If you are stuck anywhere then feel free to ask. 

Regards

Anisha Bahukhandi

Technical Content Writer