User blog comment:B214/Talk About What You Want! 3/@comment-99.101.204.248-20141127062737/@comment-16281343-20141130215241

Think of it as this. (I will be using terms that aren't technically correct for easier understanding of those who don't.)

Different pieces have different "capacities". Pawn pieces can reincarnate people with "power" equal to or lower than 10. When Rias found Issei, she had 8 Pawns, 1 Knight, 1 Rook, and 1 Bishop and Issei's "power" at that time is less than or equal to 80. Rias doesn't have any individual piece that can hold 80 except if he uses all pawns. That's why Issei was reincarnated using 8 Pawn pieces. The same can be said to Saji when Sona reincaranted her using 4 Pawns.

Now with mutation pieces, they have very large capacity. How large? There's no exact number but those who have been reincarnated using those pieces are have extremely powerful backgrounds.
 * Mutation piece Rook: Surtr Second, a clone of a fire giant.
 * Mutation piece Bishop: Gasper. You already know his.