Doping of zinc blende ZnS semiconductor quantum dots with Mn is interesting for the developing field of quantum information. A potential way to enhance the magnetic and magneto-optical responses of such materials is to increase the dopant concentration. This strategy will only be successful if the spin coupling between the dopants is ferromagnetic since antiferromagnetic coupling would lead to lower magnetic activity. In this work, we study the spin coupling and the energetics of Mn clusters by means of density functional theory calculations. We find that the spin coupling between nearest neighbor Mn dopant spins is moderately antiferromagnetic, while it becomes negligible already when the Mn atoms are in second nearest neighbor position (5.4 Å). Furthermore, the structures where the Mn atoms are clustered are more stable over the more homogeneous distribution of Mn dopant atoms. Our findings hint at a potential shortcoming of the strategy to enhance magnetic and magneto-optical responses of Mn-doped ZnS quantum dots by increasing the dopant concentration.