While an average lemon has 5 carbs, lemon juice is so concentrated that a little goes a long way! Especially once your keto palate is happening, and more subtle amounts of certain flavorings are satisfying.
Found this no-carb organic lemonade tonight, and it looks like a convenient refreshment once summer comes. Granted, these are kid-size juice boxes rather than family-sized or Lemonade Lover size - but, mighty convenient still. I love lemonade but had given it up thinking that a no-carb version wasn’t possible. But apparently, it is! WOOT 
https://www.amazon.com/Leaf-Love-Organic-Sugar-Free-Lemonade/dp/B00M9HI5IG/ref=sr_1_38_sspa