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. According to ASRock, the FM2A85X Extreme6 supports memories up to 2,600 MHz.
The ASRock FM2A85X Extreme6 has four memory sockets. Since DDR3 memory modules can be found in capacities up to 8 GB, you can have up to 32 GB with this motherboard if you use four 8 GB modules.
In order to enable the dual-channel mode, you must install two or four memory modules. On the ASRock FM2A85X Extreme6, all memory sockets are black. In order to enable the dual-channel mode when installing two memory modules, you have to skip one socket, i.e., install the first memory module in the first socket and the second memory module in the third (and not in the second) socket.