The Pentium N3700 CPU can access DDR3L (low voltage) 1,600 MHz memory, in dual-channel mode. According to ASRock, the N3700-ITX supports 1,066 MHz or 1,600 MHz memories, but not 1,333 MHz modules.
The motherboard has two SODIMM (“laptop memory”) sockets, being compatible with both low voltage (1.35 V) and standard (1.5 V) memory modules.
You can install up to 16 GiB on this motherboard if you use two 8 GiB modules.
Obviously, you have to install two memory modules in order to enable dual-channel access, which is recommended if you want to achieve the best performance possible.