It should be three surviving Targaryens seeming that Aemon is still a Targaryen even though he has taken black

No. When you take the black you're not part of your old House, plus when you're a Maester you're not. Given that Aemon didn't take part in the rebellion at all, the rebellion didn't "leave" him at the Wall. The emphasis is on Daenerys as "the last Targaryen".--The Dragon Demands (talk) 23:50, December 13, 2012 (UTC)