The 16 Gb DDR5-6400 memory chips from SK Hynix could be first to hit the market in late 2019, but the launch prices will be quite high, more appropriate for enterprise environments. On the other hand, the 8 Gb chips should be more cost-effective, thus more appropriate for consumer modules, which are expected in 2020.
https://www.notebookcheck.net/SK-Hynix-releases-details-on-the-upcoming-DDR5-6400-RAM-standard.410879.0.html