Recommended lemon juice to water ratio when making lemonade
According to this lemonade recipe Best Lemonade Ever rated by 3K people, averaging a rating of 5 stars, the ratio is

1 ¾ cups white sugar
8 cups water
1 ½ cups lemon juice

Since you're using 2 cups of sugar, I recommend you follow the recipe:

2 cups white sugar
9 cups water
1 ⅔ cups (27 tbsp) lemon juice

As for lemons to lemon juice estimate, from How Much Juice Can You Get From One Lemon?:

1 Small Lemon (4 oz.) = 3 tbsp fresh juice.
1 Medium Lemon (5 oz.) = 4 tbsp fresh juice.
1 Large Lemon (6 oz.) = 5 tbsp fresh juice

So...

using small lemons, you'll need about 9
using medium lemons, you'll need about 7
using large lemons, you'll need about 5