A single NAND die is only divided into two or four planes. It's a function of how many copies of the peripheral circuitry for accessing the array are included,

A single NAND die is only divided into two or four planes. It's a function of how many copies of the peripheral circuitry for accessing the array are included, not how many layers are in the 3D NAND array. More planes means the die can do more things in parallel (subject to constraints). A drive with 8 dies each having 512Gbit capacity divided into four planes per die will perform almost as well as one with 16 dies of 256Gbit divided into two planes, other things being equal (eg. number and speed of the channels between the SSD controller and the NAND, page and block sizes and access times, all of which are subject to change at the same time a generational change increases die capacity and number of planes).