I'll provide an example to be clear ^^
Let's say it's round 1 : magneto places 1 construct, he gains 1 power from power phase +1 for the construct
Round 2 : he places another construct (2 in play), he gains 1 power from power phase +2 for the constructs
Round 3 : he already has 2 construcs in play and so he places none, how many power does he get ?