The edits that Kalyan is refering to are in the XSD file for the DOM that you are talking about. If it is the Main DOM that you are referring to then you will want to extract the form files to a new folder, open the "myschema.xsd" file in a text editor, find the line that references this particular element, and make the appropriate edits. If you are not familiar with XML schema files you probably wnat to Google around for some more general information to help you find your way around the file.
Another way to do this would be to use conditional formatting or data validation that counts the number of rows that have been inserted and either disallows adding new rows or displays an error message upon inserting another row and then deletes the last row added.
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge