The usage of additive manufacturing techniques has increased dramatically in recent years. Fabricated parts are no longer simple prototypes, but rather structural components whose mechanical characteristics must be understood before printing. One of the weaknesses of 3D printing is the high variability of dimensional, geometric, and mechanical properties, which is due to the combination of various printing parameters, including the number of walls, roofs, and floors, filling patterns, and printing layer thickness. This study aims to predict the mechanical properties of onyx printed parts as a function of the number of walls and a solid pattern through an analytical approach based on the rule of mixtures and numerical finite element simulation. The influence of the number of walls on the mechanical properties of onyx printed parts was characterised by uniaxial tensile tests. The results show that walls have a significant impact on the final mechanical properties of the parts. The study found that the higher the number of walls, the greater the mechanical properties of the parts. The rule of mixtures approach allowed us to predict the mechanical properties with good accuracy, with prediction errors observed ranging from 1% to 10% depending on the number of walls in the parts. The numerical simulation using finite elements was carried out using the properties of the walls and the solid pattern obtained from tensile testing, enabling a comparison between the experimental test and the rule of mixtures. The results show that the mechanical properties obtained by the rule of mixtures and numerical simulation are consistent with the physical tensile test.