How do I create sequential invoice numbers in numbers?

You could concatenate an invoice number based on the row it is in the table. For instance:

You'd need to choose the offset (1000 in the example above) to match your needs. Once the formula is added to all the rows, the formula will be entered automatically when you add a new row.

There is no built-in feature that will open an invoice template and automatically give it the next available "invoice number". Nothing in Numbers knows what the previous invoice number was nor does it have the ability to automatically put a number into a cell when a template is opened. This is something a database would be good for.

Since you have given no specifics of your setup it is hard to give a meaningful answer.

There are many ideas about how to use Numbers for billing and invoicing in this video:

SG