The NCS has a -1 charge, Ni is 2+, so 4 K are needed to balance the 4- charge. The K and water will sit in the crystal lattice associated with the central octahedral complex [Ni(NCS)6]. In this case isomerism may occur with the following: K4[Ni(H2O)(NCS)5](SCN).3H2O, K4[Ni(H2O)2(NCS)4](SCN)2 .2H2O etc.
Hope that helps.