In that situation you will be sweetening your beer. I'm not sure if that's what I'd want to do, because then you may end up with a cloying or syrupy-sweet beer.
I have not brewed a maple beer like this before, but my guess would be a ballpark of 16 oz of syrup as a starting point for a 5-gallon batch. But, like I said, I would much prefer to let the added syrup ferment out in secondary than add it at kegging. Backsweetened beer just doesn't sound that appealing to me.