I have done it on a per bottle basis, when I no longer had any other stuff sitting around, as a rule of thumb, a big squirt per bottle is too much and a little squirt is not enough.
I imagine priming with maple syrup is alot like priming with honey, which has its difficulties and requires a little fancy math.
According to John Plamer:
You can prime your beer with any fermentable that you want. Any sugar: white cane sugar, brown sugar, honey, molasses, even maple syrup can be used for priming. Honey is difficult to prime with because there is no standard for concentration. The gravity of honey is different jar to jar. To use honey, you will need to dilute it and measure its gravity with a hydrometer....