It is a Ratio. If your nutrient ask for 10 ml per 4 liters and you want to mix 20 liters then it would be 50ml. It is just like mixing oil into gas for a two stroke engine. 50:1, 20:1 what ever. It does not matter how many plants you have. Stick with the Ratio.
4liters:10ml=20liters:50ml
Myself, if my plants eat up nutrient soup, I never add water AND Nutrients. If I MUST, I just add water. Other then that, I mix a new batch.