Try this, it's a recipe for homemade lemonade concentrate. But it seems to me that if you have to go out and buy all those lemons, you might as well just pick up a can or two of frozen concentrate.
In a saucepan, stir together 1½ cups of white sugar (375 mL) and 1 cup (250 mL) of water. Heat until the mixture boils and the sugar is dissolved. Let cool.
Meanwhile, juice 8 medium-sized lemons until you get 1½ cups (375 mL) of juice. You may need to strain your juice if you have pits or too much pulp. Once the sugar syrup has cooled a bit, add the juice, stir, and then put in a plastic container with a lid in the freezer. This is about 3 cups (750 mL) worth of concentrate. Freeze solid. Can be stored for up to 6 months.
On a hot sunny day when you feel like a lovely drink, defrost the concentrate. To make a big jug, mix the entire 3 cups (750 mL) of concentrate with 3 cups (750 mL) of water. Fill glasses 3/4 full, and top with ice cubes or fizzy soda water. To make only one glass, combine equal parts concentrate with water, and keep the rest of the concentrate in the fridge for later.