Rotterdam was mentioning on the WCS NA stream that he could power 14 gates off 1 pylon, but that 15 "might" be possible. Alas, I have no SC2 so I cannot see if he is right or wrong.
16 seems to be the max. credits to /r/starcraft
It is possible for 15 to be powered off one pylon. Ignore that other green blob, it only has 14 :)