It is possible to attach up to 64 attachments to a single message; however, this is strongly discouraged.
To ensure the batch processing portion of the conversion process runs efficiently for outgoing faxes, it is recommended that you don't attach more than 32 attachments to a single message.
A further limitation can be set by the mail server or IIS SMTP server which can restrict the size of the message or the number of attachments in the message.