As I can see '/skyisland new' allows you to create 63 islands in addition to the starting island. The join and located subcommands take a number from 1 to 64. These don't seem to be shifted to the 0 to 63, which are the islands' actual indices. Consequently, it is impossible to join the original island and therefor to have the full 64 islands.

The bug here is that the new command doesn't allow generating the 64th island (I used range not rangeClosed). The world spawn is not supposed to be island zero and instead should be a landing zone for new players when the skyIsland command is used. The spawn structure can be set using a "spawn_platform.nbt" structure file while the islands use a "sky_island.nbt" structure file.