How to I keep the estimate number on the invoice, I have created from the estimate.

Hello Wright, 

Welcome to the Community page, 

If you go to the main cog wheel> click on custom form styles> click on the template you use on invoices> click on content> click the top box> then at the bottom you will see one that says custom field if you click that and add it on there it will then show up for you and your customer.