User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170506225314/@comment-29963364-20170506232005

When a King is reincarnating a being to be their Servant, the number of Pieces used is determined by the Servant's strength/power and/or potential in comparison to the King's strength/power  at the time of the reincarnation ritual.

Considering how powerful Issei has become, I doubt anyone he reincarnates will need more than one Evil Piece, unless it is someone exceptionally strong/powerful, or has potential (like when Issei was reincarnated).

I know that it was stated that Bova and Ouryuu are worth 3 and 5 Pawn Pieces respectively, but those values were determined by the "special rules" of the World Rating Game Tournament. I don't think they were using Issei, who is the King, as a comparison to determine their values.