See also The Cleanest Protein Powders.
Every protein powder I’ve ever seen comes with a scoop calibrated to the nutrition facts label on the container, even if the size of the scoop varies from brand to brand. If the label says “Serving size: 1 scoop” and one serving contains 20 grams of protein, 2 grams of carbs and 1 gram of fat, that’s what’s in the scoop. And that’s what you take.
Some brands contain more protein than others, and some “protein powders” are more like meal replacements, containing a mix of protein, carbs and fat. The majority of the powders (and scoops) deliver about 20 to 25 grams of protein per serving, which works well for a smoothie.
If you’re using your protein powder simply as a protein source, you might consider finding one that is largely protein with only a few grams of carbs or fat. And if you want to turn that protein drink into a homemade meal replacement, you can add berries for carbs, ground flaxseed for fiber and a spoonful of flaxseed or fish oil for fat.
Most people don’t need more than 20 to 30 grams of protein per meal, so the “scoop serving” in most powders should be just fine.