In the paper we analyze the generation of the runaway electrons (RE) in multicomponent JET boundary plasmas during a preemptive disruption current quench phase, when considerable amount of Ar is injected into boundary region. The impact of Zeff on the onset of the RE generation as well as on growth rates of primary and secondary REs is considered for multicomponent JET boundary plasmas in CFC and the ITER-like wall (ILW) discharges. Two electron density boundaries can limit the occurrence of the REs depending on concentration of impurities. An upper density limit is related with the increase of friction force at higher densities ("Rosenbluth density limit"). Another limitation could occur due to the excitation of helicon waves by the REs, which prohibits the avalanche development and restricts for given toroidal magnetic field the RE occurrence at low densities ('Helical density limit'). These density boundaries of the RE occurrence as function of impurity concentration are derived. The important role of Zeff on the RE generation in the CFC and metallic JET wall cases may explain the difference in the RE occurrence in both discharges.