Assume there are 100 drops/teaspoon.

There are 3 teaspoons in a tablespoon.
So 3 x 100 drops = 300 drops in a tablespoon.

There are 16 tablespoons in a cup.
So 300 x 16 gives 4800 drops in a cup.

There are 16 cups in a gallon.
So 4800 x 16 gives 76,800 drops in a gallon of sap.

It takes 40 gallons of sap to make one gallon of maple syrup.
So 76,800 drops x 40 gallons gives 3,072,000 drops.

So it takes 3,072,000 drops of sap to make one gallon of maple syrup.