It can't do a set number of rings, it is either answer immediately (Dedicated fax line) or Manual answer (Shared phone line).
You would likely need to look into getting DRPD on the line if it is shared (Assuming phone companies stiull do such a thing)
Does the WC6515 AutoAnswer for fax have the ability to adjust the number of rings or time before the device answers the line? There is no mention of it in the user-guide.