You must see the 15 bridges! Adjacent to Rembrandtplein is Thorbeckeplein; walk south to Herengracht and stand on the bridge.
Turn your back to the Thorbeckeplein and you will see six arched bridges across the Reguliersgracht. Then look down Herengracht to your left and you see another six bridges. Look right, and there are two more ... 14 so far? The 15th is the bridge you are standing on!