of each (24 total) would be much easier to work with. this is a personal preference and you could use any amount you’d like.
now for the length: the strands of the tails i make are roughly 17-18 inches long. that makes them long enough to be fun, but not quite long enough to drag the ground for most kids over 3 years old. aurelia is about 3’4″ and you can see how long they are on her here. you may need to adjust this length if your child is shorter, but by my logic a half yard of fabric is 18″ so why mess with fate?
stopping my cuts about 3/8″ 