There are a total of 58 players . The DA members are 28, so if you meet all them you are almost halfway finished with this. The teachers count, as do the Gobstone champions, the Snap champions and the Chess champions. I am pretty sure that the two girls from the Slytherin Portrait count as well, but I am not sure. I am also unsure if Remus and Tonks count. Just to be safe, make sure you talk to them. When I played through a second time I didn't talk to them at Grimmauld Place, but when I beat Voldemort it unlocked. Which may have been caused from the first time I played I wasn't able to do McGonagal's mission until after I beat the game, then it unlocked. Food for thought. You don’t have to talk to everyone in the game so don’t bother, as you will usually just get smart alike remarks or hello’s.