Mapped Content Controls - That is what you are currently using under a different name. The so-called document properties under Quick Parts are a special case of these built into Word beginning with Word 2007.

See Repeating Data Using Document Properties Content Controls and Other Mapped Content Controls which describes what you are already doing and leads into Greg Maxey's tool for mapping any content control. I do not believe there is a practical limit to the number you can use with his tool. You can also have access to drop-down, rich text, and date-picker controls. While you do not need Greg's tool for mapping other content controls, it makes the process much easier.

Other methods are discussed at Repeating Data (Populating Multiple Like Fields) but the mapped content control is the easiest so long as you do not have to use the form in versions of Word prior to Word 2007. If you only want to change the value in one place (and have that reflected elsewhere) an even simpler method discussed on this page is "Simple Linked Content Controls."