This is a really good question, since there are so many different protein shakes on the market, and everyone has a different fitness goal. I say different but most people are looking to do 1 of 3 things, lose weight, add muscle, or maintain.
If you’re looking to lose weight then you want a lower calorie protein shake where you would just add water or low calorie fluid to blend it with, I’d recommend per 20-25 gram scoop 90-150 calorie max. The math behind protein is simple, per gram of protein it natural yields 4 calories. This is without adding any substance for taste or texture. So naturally 20 grams of protein yields 80 calories, 25 yields 100 calories. The shake I use has 120 calories per 24 grams of protein which is very good and lean. You’ll find some proteins that focus a-lot on flavor or texture or even building muscle or muscle mass and these should be used for those goals.