User blog comment:B214/Question and Answers 6/@comment-9595671-20160715025503/@comment-24.113.252.213-20160715030821

1.) A Queen Piece = 9 Pawn Pieces // a Rook = 5 Pawns // a Bishop = 3 Pawns // a Knight = 3 Pawns // a Pawn = 1 Pawn.

Each person can only possess one Piece of each type. If Rias could have reincarnated Issei with 2 Rook Pieces, or 1 Queen Piece. And yes, someone's strength does make a difference. If Rias was stronger, she could have reincarnated Issei with fewer Pawn Pieces.

2.) Issei's Pieces changed to Mutation Pieces because they adapted to Ddraig's power (Illegal Move Triaina and Cardinal Crimson Promotion). I don't know if Mutation Pieces are a "one piece fits all" kind of thing. It is stated that Mutation Pieces are used when Multiple Pieces of the same type need to be used.

3.) The King Piece was the original way a Devil marks themselves as the "King" of their peerage. But after it became illegal (for variouse reasons), they now use a monument which simply "registers" them as the "King" of the peerage. It's like registering yourself as the manager/leader of a team.