Here's the thing to remember about shakes:
1) They need to be used as a means to 'recover' used up energy, and to fuel muscle growth.
2) They need to be used as a 'meal replacement' as opposed to an 'extra source' of protein.
Protein is what you need give your body so that it can build muscle. It will only work though, if you use it with a good exercise plan. If you are trying to bulk up a little, i would recommend any products with tags such as 'X-plode' or 'Whey', as these supplements aid the body in storing as much muscle fuel as possible.
Also, never drink shakes with milk. Water only people. Shakes + Milk = milkshakes. Which is fatty! But, in your case, i might actually recommend you drink it with milk, as opposed to water, because you WILL gain a little bit of weight if you do this.
As for how much of it to drink, well that depends on your lifestyle. I am at the gym twice a day. I go before work, and do strictly cardio (running rowing and swimming). Thus, i use the shake as a form of meal replacement after i am done to replace the lost energy. I then hit the gym straight after work, and focus strictly on weight training. Once i am done, i drink a shake and feel my muscles build as i sleep!
If you are far more active than that (rugby practice or whatever) I would say 3 a day is good, as long as you can afford to keep it up financially that is.
If you are really struggling to pick up mass, then eat more. Red meat is fantastic for mass gain, as well as cheese and plenty of milk. Also, try to cut down on cardio, and focus more on weight training. Don't sit and do 15 reps of a weight you can handle because remember, the heavier you lift, the more muscle is needed, and thus the most growth will happen. Low reps of heavy weights are the best way to gain in size, rather than shape.
