The range of servings is from 6-18 glasses (and up), with the recommended average being 10 glasses or servings of a standard water glass (8oz). We know that each serving has 2 tablespoons of lemon juice in it, but what we often don't know as it's a variable is the size of the lemons, or the amount of juice we can get out of each one.
There are small lemons, true. And huge ones too. But the average lemon, is average because they are easiest to find. So let's assume the average, which happens to give about 2 tablespoons of juice per lemon. If it gives more or less, what I do is take the drink at the size the good lord gives me. I'll just match the maple syrup amount to the lemon juice amount. So if I get 1.8 tablespoons (by scientific eyeball method), I'll add the same amount of maple syrup. If it's 2.4, it's a super sized serving for me today.
I will keep an eye on if most drinks are larger, and pushing me to take more lemonade than I planned for, but really the difference is insignificant. I mean, maybe I end up with 9 servings 1 day and 11 the next. It matters not!
So assume 10 lemons per day.