Autoflowers will grow under any light sequence - even 12/12 but will perform poorly in that scenario. Similarly, they will grow under 24/0 but will be stressed a bit (although honestly not much)... Ideal is to grow either under 20/4 or 18/6. For me, it depends a lot on whether or not I can keep up the temps in my tent during the winter... When I first started growing autos (and they're all I grow), I had to grow them under 24/0 because the temperature fluctuations were too drastic... This year, the winter temps haven't been as low and so I'm able to grow under 18/6 and my electric bill is thanking me profusely as are my plants.
Good luck! And always come back if you have other questions during the grow! Enjoy the journey!