The last time Sony put 16GB RAM on their smartphone was the Japan-only Xperia 1 iv Gaming Edition. I won't be surprised if 16GB RAM option is also limited to Japanese market only.
The Xperia 5 V has now been seen on Geekbench shortly before its official release. While the smartphone has been benchmarked with 8 GB of RAM, there are rumours that Sony is also planning to offer a 16 GB of RAM edition for some markets.