The capability of counting targets (people or things) in a monitored area is important for emerging wireless applications. To this aim, passive systems that rely on signals of opportunity and device-free targets are preferred to active systems that rely on dedicated or personal devices for preserving privacy and reducing implementation costs. This paper develops a framework for design and analysis of device-free counting systems via OFDM signals of opportunity. In particular, counting techniques based on model order selection are proposed. Preliminary results show the effectiveness of the proposed techniques in simple use cases.