How much protein powder per day
Protein powder is optional. The ISSN range for exercising people is about 1.4 to 2.0 grams of protein per kilogram of body weight per day from food and supplements combined. A large meta-analysis found extra protein helped lifters, with little added benefit once daily intake was already high. One scoop is not a target. The gap between what you eat and that range is the scoop.
Whey protein, casein, and other powders are foods with a nutrition label. Protein supplementation in the trials means "more daily protein," not a specific brand.
The range the position stand uses
The ISSN protein stand puts exercising people around 1.4 to 2.0 grams per kilogram per day (PMID 28642676). A 70 kilogram adult at the low end is about 100 grams of protein from all sources, not 100 grams of powder. Morton's meta-analysis (PMID 28698222) found gains from added protein during resistance training and a flattening of the benefit as intake rose, around 1.6 grams per kilogram in that analysis.
How to use a scoop
Add up the protein you already eat. If you are short, one scoop of whey or casein protein can close the gap. If you are already in the range, another scoop is calories. Older adults who eat little protein are the group where the gap is most worth closing. Kidney disease is the situation where a high protein target needs a clinician, not a tub.