It depends on yield. If it's very high, you save money using only 6 chiplets. If it's not as great then using 8 lets you downbin the 64 core parts that were def

It depends on yield. If it's very high, you save money using only 6 chiplets. If it's not as great then using 8 lets you downbin the 64 core parts that were defective into the 48 core slot. You can't do both in the same SKU because they have different performance, e.g. different amounts of L3 cache & memory bandwidth.