i think 6?
Christi and Kelly had often said they were signed to six-year contracts.
The real answer for the show is: "As long as Collins Avenue and Lifetime think they can keep making good money from a show they title Dance Moms."
If Abby and some others were willing to do a seventh season, and the network and producers thought ratings would be good enough, it would keep going. (If ratings suddenly crashed, it could be over after Season 5.)