The only required fields are:
- Issue Date
- Client name [Must exactly match any pre-existing client or one will be automatically created]
- Description
- Total gross amount
So account number in this case would only be used to match with clients on your account. And the gross figure isn’t actually used at all - it would just be net, VAT rate and VAT amount.
There’s a breakdown of the fields at the following link, which may help:
Importing sales and purchase invoices