The Truck Factor is a simple way, proposed by the agile community, to measure the system's knowledge distribution in a team of developers. It can be used to highlight potential project problems due to the inadequate distribution of the system knowledge. Notwithstanding its relevance, only few studies investigated the Truck Factor and proposed ways to efficiently measure, evaluate and use it. In particular, the effective use of the Truck Factor is limited by the lack of reliable thresholds. In this preliminary paper, we present a theoretical model concerning the Truck Factor and, in particular, we investigate its use to define the maximum achievable Truck Factor value in a project. The relevance of such a value concerns the definition of a reliable threshold for the Truck Factor. Furthermore in the paper, we document an experiment in which we apply the proposed model to real software projects with the aim of comparing the maximum achievable value of the Truck Factor with the unique threshold proposed in literature. The preliminary outcome we achieved shows that the existing threshold has some limitations and problems.