AMD CPUs have an embedded memory controller, meaning that it is the processor, not the chipset, that 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 FM2 processors supports DDR3 memories up to 1,866 MHz. FM2+ CPUs memory specifications are not informed by AMD to date. According to ASRock, the FM2A88X EXTREME6+ supports memories up to DDR3-2600.
The ASRock FM2A88X EXTREME6+ has four memory sockets. According to the manufacturer, it supports up to 64 GiB if four 16 GiB memory modules are used.
In order to enable the dual-channel mode, you must install four or two identical memory modules. If you are using two modules, you must skip one socket. It would be nice if the manufacturer used sockets of different colors in order to show which ones to use with two memory modules.