User blog comment:B214/Volume 23 discussion blog/@comment-1359322-20170318060342/@comment-10330822-20170319023343

I don't think raw power really affects the piece requirement. I think it has more to do with usability. For example, Gasper was able to be reincarnated when Rias used a Mutated Bishop piece on him, but he needed to be sealed soon after because Rias wasn't strong enough to use him.

When it comes to Saji, I feel that the 4 pieces is in relevance to Vritra's power as a whole, not just the fact he had just one piece. After all, if it mattered, his pieces would have needed to be change when he got more of them. Just like Issei's pieces needed to be altered for him to get stronger.

As for Issei, when he was tapping into the real power of the Boosted Gear, they needed to mutate 4 of his 8 pieces. Likewise, 8 is the maximum you can use, kind of like how most of Sirzechs' OP squad uses 2 normal pieces per person.