We aim in this paper to investigate a preventive maintenance scheduling with imperfect repair for a manufacturing system. The considered system is composed of several parallel leased machines. It has to satisfy a random demand over a finite horizon. Machines are subject to failures that are strongly correlated to the production rates of each machine. The objective is to study the impact of imperfect preventive maintenance on maintenance plan for a leased system characterized by a variable number of machines. We propose hereinafter a model which allows minimizing the total maintenance cost by obtaining the optimal number of imperfect maintenances for each machine. A numerical example is given in order to validate the robustness of the developed approach.