It has never been stated which number Zaraki Kenpachi is. However it is safe to assume that Zaraki Kenpachi can be any number starting with 10 and moving upwards. This speculation is confirmed by the Turn Back the Pendulum arc where Shinji stated that Squad 11 had recently gotten a new captain who is the 10th Kenpachi. However it was never stated how this Kenpachi became a captain which would have confirmed whether or not it is Zaraki Kenpachi.
It's quite probable that Zaraki is the 10th Kenpachi, as Ikkaku and Yumichika came to Seireitei after he did, and during the Ikkaku vs Ichigo fight, Ikkaku remembers Urahara as a Captain, implying that he had to have been in Seireitei at the time of the Turn Back The Pendulum arc.