This work presents an S₂₁-based 2.4 GHz microwave sensing system with a 3D-printed dielectric lens for automatic passenger counting in aircraft applications. The proposed system employs a pair of microwave antennas forming a transmission sensing link across the passenger pathway, where the presence of a passenger perturbs the transmitted microwave signal and produces a detectable variation in the transmission coefficient, S₂₁. A hemispherical dielectric lens fabricated from acrylonitrile butadiene styrene (ABS) is integrated with each antenna to improve the directional radiation characteristics of the sensing link. The measurements show that the lens increases the average realized gain from 2.26 to 4.02 dBi while maintaining operation at 2.4 GHz. The sensing system is integrated with a NanoVNA and a MATLAB-based processing platform for sequential S₂₁ acquisition, human-event discrimination, automatic counting, and automatic display updating. Experimental responses obtained from humans and representative non-human materials, including plywood, plastic, and steel, demonstrate distinguishable S₂₁ characteristics under the investigated conditions and provide the basis for an experimentally derived human-response detection window. The complete system was experimentally evaluated in an actual Airbus A320 aircraft using 30 participants over five repeated counting trials. The system achieved a mean counting accuracy of 90.67%. These results demonstrate the feasibility of the proposed microwave sensing approach for aircraft passenger-counting applications.