I finally got my ASRock J5005-ITX motherboard and can confirm that it is able to handle 16GB of RAM. Although Intel's web page says this CPU supports up to 8GB, I have successfully installed Ballistix 2x8GB SODIMM kit (model BLS2C8G4S240FSDK) and it has got correctly recognized by BIOS and Ubuntu OS.
There are also other people reporting that this CPU supports 16GB. For example in this review Intel's NUC built on J5005 works with 16GB of RAM. There's statement that RAM modules need to be 2400MHz ones. This not necessarily has to be true as ASRock maintains RAM compatibility table for mentioned motherboard where 2133MHz modules are also listed.
Edit note: A reader contacted me regarding compatibility issues with his Kingston HyperX Impact 8GB - HX424S14IB2/8. He bought two pieces for dual-channel but motherboard behaved like the RAMs were damaged. Even when using only one of them. Finally what helped for him was the BIOS upgrade. Now they run smoothly in dual-channel. So keep that in mind that flashing the newest one to your J5005-ITX can add support for your RAMs.