The use of Weigh-in-Motion (WIM) systems for vehicle's direct enforcement depends, first of all, on their high and constant accuracy under variable measurement conditions. This work concerns improving accuracy of weighing results obtained in WIM systems. The method commonly employed for achieving this goal is to increase the number of axle load sensors at the weighing site. The objective of this study is to compare the accuracy of Weigh-in-Motion systems equipped with different numbers of axle load sensors. The paper presents the results of in situ tests and simulations. Experiments were carried out on 16-sensors system and were supplemented by results obtained by simulations using multibody dynamic software. For accuracy assessment of weighing systems, two criteria were used: the 95%-percent relative error and the reliability characteristic. Quantitative relationship between the number of axle load sensors and weighing accuracy was estimated. Optimal number of sensors was evaluated for multi-sensor systems which is a step forward in implementation of WIM systems for direct enforcement of overloading.