Can order number be left empty on import? no, it is required. See the import requirements for a SO or Quote in BW

Also there are maximums to the quantity of SO and Invoices per customer

3,000 Quotes
6,000 SO
12,000 Invoices

Hi Ignacio,
Sales order number is a required field. When you reach 999999, you will need to start your ordering system over again. There is no other workaround at this time.

There is a checkbox 'Reassign all quote and order numbers'. Select this check box to renumber the imported quotes and sales orders starting with the next available number showing in Sage BusinessWorks. If this check box is cleared, the imported quotes and sales orders use the numbers from the import file. This may cause errors during the import process because the system cannot import a quote or sales order with a number that already exists in the database.

Thank you,
Janice