Did you look in the Knowledge Base? I did and here is what I found:

To Auto-assign the next available batch number when running the VI Import job:

Use a Replace operation to assign the batch number.

Add the Batch Number (H.BatchNo) to the Data tab in the VI import job.
Select the following in import field properties:
Operation: Replace
Source: Point the field to a blank column in the source file

The next batch number is automatically assigned when the Visual Integrator import job is run.

For example:

Import into AP_InvoiceHeader:
on the Data tab, add H.BatchNo field from available fields on the left
change the column for the Replace action (if needed) to point to a blank column in the source file.