Board Thread:Episode Discussions/@comment-26005711-20150730124954/@comment-1512913-20150731122130

Except Naruto and Sasuke came directly after Hashirama and Madara, there were none in between because of Madara reviving himself and taking Asura's chakra, so Indra and Asura's chakras couldn't incarnate again while he was still alive. Right after Madara's death, Naruto and Sasuke were born, the next incarnates. Before Hashirama and Madara, there had been many others, over span of about 1000 years. Average human lifespan give or take 80 years and it's likely the incarnates died before that, killing each other off... so say 50 years their average lives, that means there might have been as many as 20 other Asuras and Indras excluding the known ones.