You would probably start with one cable and increase it over time. HVDC cabels are already routinely build undersea. There is a 700km long cable under construc

You would probably start with one cable and increase it over time. HVDC cabels are already routinely build undersea. There is a 700km long cable under construction between the UK and Noreway. The longest HVDC connection is over 2000km long in Brazil. Superconductors have a maximum current, if you go above that current, the superconductor will gain resistance and the heat will destroy it. So you would still use high voltage transmission.