A maximum of 999 recipients can be associated with any fax job. If the entire spool file is the fax, then 999 recipients can be specified. If you’re using FastFax API jobs and the spool file has multiple fax jobs in it, then each fax job within that spool file has a limit of 999 recipients.
If you need to send a document to more than 999 recipients, then you need to send the same spool file multiple times. I would recommend using the FastFax Phonebooks and take advantage of the Broadcast List feature. This feature allows you to create one phonebook entry that defines up to 999 recipients. You can create multiple Broadcast Lists (of up to 999 recipients) to easily complete such a job.