Thank you for this question, AussieMinecrafter.
Languages are limited to 6, because they are meant to be those you can provide subtitles in, by transcription or translation, and most people cannot do that in more than 6 languages.
However this only matters in some teams. If you want to provide subtitles in a language not in your 6 listed ones on a public Amara video page, you can.
If you want to raise this issue with Amara tech staff, please create a ticket: it will go to them, and they can give you a better answer. You could also use the ticket to request the addition of Arrernte to the list of subtitling languages.