Hi Tracy,
Am I correct to assume that you're trying to do this without writing any code? If you are writing some code, you can refer to the Asset Tracker sample that ships with InfoPath. If you are trying not to use code, you should be able to create a rule that increments a counter stored in a secondary XML data file (data connection) and refer to that field when a new row is added.
Patrick Halstead
Project Manager at Qdabra