More reeds make for greater versatility. Your Geographic Area. People have even given them away. That is because they are still relatively common here. However, on the West Coast they are far less common and therefore are worth more.
Age is fairly easy to gauge from cabinet styles.
The materials used can sometimes help too. Parlor organs made of oak do not date before the late 1880's. Some makers dated their instruments on the back panel, on a label inside the instrument, or on the back end of the keyboard. See the picture gallery included in this website for examples of these different types of reed organs. There were over 500 makers of reed organs in the United States during the nineteenth century, and little or nothing is known about some while a great deal is known about others.