Dense Networks and large MIMO are two key enablers to achieve high data rates towards next generation 5G networks. In this context, we study user association in an interference limited Multi-user MIMO Small Cell Network. Extending on our previous findings, we derive explicit expressions for the optimal ratio of the number of antennas at the base station to the number of users that can associate to a base station in such a Network. This expressions are used to compute the actual number of users that can associate for a given interference level and other system parameters. Simulation results and numerical examples are provided to support our theoretical findings.