This question could be taken in two different ways. It could be seen as wondering who has the number 2 as their placement in the Troupe or who is the second in charge in the group. When it comes to the person who holds the number 2 position, that is Feitan Portor. Feitan is numbered two. Ironically, he is also the same answer to the other side of the question. He is formerly the interim leader for the group when Chrollo is not around. As such, if there is anyone who would be considered next in charge, it is Feitan. Now, if we are talking about the second most powerful member of the Troupe, that will obviously go to Hisoka. Illumi is another strong member but I think that Hisoka would defeat him in battle.