If we're going with the theory that Asura is a Jinchuuriki, probably has the Four-tails sealed in him if we're just counting the amount of tails. If you count the four extra arms he has, it's the Eight-tails. If we're counting all six arms too, then it's the Juubi, which would be somewhat impossible.
But what we do know is that it's either something special that Senju have, or just something plain special about himself. Hagoromo did say that he worked hard enough for his chakra to bloom.
MangaPanda translation: "By suffering and training hard, the chakra within his body bloomed. And he gained the same power as his brother."
MangaStream translation: "Through harsh training, the chakra inside his body bloomed and he found power on the same level as his brother."
Apparently he was special enough to make five Tailed-beast balls at once. But anyways, this chapter was pretty informative. Even-though this was somewhat out of nowhere. Loose ends are being tied up, so expect for the series to be done in about 2-3 years.
