Судя по спецификации материнки, 8 планок поддерживается для процессоров с 6 ядрами и выше. А Intel Core i7-7740X - 4 ядра
Intel® Core™ X-series Processors (4-core)
* Refer to www.asus.com for the Memory QVL (Qualified Vendors Lists).
* Hyper DIMM support is subject to the physical characteristics of individual CPUs.
4 x DIMM, Max. 64GB, DDR4 2666/2400/3600(O.C.)/4000(O.C.)/4133(O.C.)/2133 MHz Un-buffered, Non-ECC Memory
Intel® Core™ X-series Processors (6-core above)
8 x DIMM, Max. 128GB, DDR4 4000(O.C.)/4133(O.C.)/2133/2666/3600(O.C.)/2400 MHz Non-ECC, Un-buffered Memory