A family of Microsoft word processing software products for creating web, email, and print documents.
You'll need a macro to do this. The macro can be selected as the "exit macro" in the properties dialog of the form field in which the number of rows is specified, so the macro will run when the cursor leaves that form field.
In the code of the macro, you'll have to turn off the forms protection, insert the requested number of rows, and then turn the protection on again.
In order for the macro to know where to insert the rows, you'll have to use some method of locating the correct table. For example, if the table is the first (or only) table in the document, you could have the macro add rows to ActiveDocument.Tables(1). Or you could insert a bookmark named "TestTable" around the table, and the macro would add rows to ActiveDocument.Bookmarks("TestTable").Range.Tables(1).
If you need more help with the macro, post back.