Good evening, @Brp. I'm here to lend a helping hand.
We can create a Custom Form Template for your Estimates to show the Invoice Number. I've provided the steps below on how to do this with ease:
You can now open an old Estimate or create a new Estimate with this template. All you have to do is click on Customize in the footer when you have the Estimate opened, then enter the Invoice No. in the field. After doing this, this template will automatically populate when you create this form.
Let me know if you have additional questions or concerns. Have a great day!