In this paper we study the optimal placement and optimal number of active relay nodes through the traffic density in mobile sensor ad hoc networks. We consider a setting in which a set of mobile sensor sources is creating data and a set of mobile sensor destinations is receiving that data through multihop wireless paths. We make the assumption that the network is massively dense, i.e., there are so many sources, destinations, and relay nodes, that it is best to describe the network in terms of macroscopic parameters rather than in terms of microscopic parameters. A simple one-dimensional scenario is used to introduce the problem. We solve the two-dimensional scenario where the mobility of the nodes is deterministic and when it follows the brownian mobility model.