Intel socket LGA2011-v3 CPUs have an embedded memory controller, meaning that it is the processor, not the chipset, which defines what memory technologies you can have and the maximum amount of memory that is possible. The motherboard, however, may have a limitation as to how much memory can be installed.
The integrated memory controller from socket LGA2011-v3 processors officially supports DDR4 memories up to 2,133 MHz. According to ASRock, the X99 Extreme6/3.1 supports memories up to 3,200 MHz.
One of the most important features of the socket LGA2011-v3 processors is the support for the quad-channel memory architecture, which allows the memory to be accessed in 256-bit mode for higher performance. Since each memory module is a 64-bit entity, four memory modules are needed to enable this architecture. If only two or three memory modules are installed, the memory will be accessed under dual- or triple-channel architecture, respectively.
The ASRock X99 Extreme6/3.1 has eight memory sockets (four at each side of the CPU socket) and you can have up to 128 GiB with this motherboard if you use eight 16 GiB modules.
In order to enable the quad-channel mode, you must install four or eight identical memory modules. If you are installing four modules, you must install them on the black sockets to achieve maximum performance.