The per serving of the mass gainer is 1/2 scoop? 

wanna bulk? increase your calorie.
if you eat enough, u dun even need supplement.
if you don't, don't try to save your supplement, just drink.
see the label, one serving gives you how much calories.
sometimes, one serving could b 3-4 scoops.
so far most of the mass gainers have more than 1 scoop per serving unless the scoop is xtra big.

it's all up to u.
read up the supps fact.
read wat they do.
then u decide whether to take it or not.
for me if u r a beginner, get the basic things like protein n multivits.
then from there u can move on to other things like pre wo NO, pwo recovery, etc.