Hello Katrin. A classic and well-balanced ratio for traditional homemade lemonade is roughly one part freshly squeezed lemon juice to four parts water, along with sweetener to taste.
Since one liter is equal to about four standard cups of water, you will ideally want about one cup, or 240 to 250 milliliters, of lemon juice to achieve a crisp, refreshing flavor. On average, a medium-sized lemon yields around 2 to 3 tablespoons of juice (roughly 35 to 45 milliliters). Therefore, you will typically need between 4 and 6 medium lemons for one liter of water.
If your lemons are firm or thick-skinned, plan on using around 6 lemons, whereas softer, juicier lemons will usually reach that target with just 4 or 5. To complement this proportion, mixing in roughly 150 to 200 grams of sugar (preferably dissolved into a warm simple syrup first) will give you a perfectly balanced drink without any gritty sugar residue at the bottom. I hope this helps you make a wonderful batch.