As a general rule, we would not recommend using more than two servings of soy protein powder each day. This should be sufficient for most training or weight loss programs. If you're training as an elite athlete and require further supplements we would advise using a blended plant-based product from more than one protein source.
Adults need 0.35 to 0.75 grams of protein per pound of body weight a day, depending on their activity level. You should not exceed 1 gram of protein per pound as this has been associated with some health risks.
If you already have a very soy-rich diet we would recommend no more than one serve of soy protein powder per day. Soy can affect thyroid function which is not a problem unless you are consuming excessive amounts of soy protein. You can always use alternatives such as rice and pea protein powder, which when taken together also provide a complete protein source.