TDC specifications are critical determinants of the range, resolution, and accuracy of SPAD-based TCSPC 3D imaging systems. In particular, the TDC conversion time plays a vital role in emerging architectures wherein TDCs are shared across the sensor array. Here, a statistical analytical model is employed to relate the number of required TDCs per SPAD array to the 3D image accuracy and frame rate, taking into account environmental factors such as ambient light and distance. The model verifies that sharing schemes, contrary to TDC-per-SPAD scheme, can improve efficient use of area and power, and it also permits TDC sharing architectural exploration to determine the number of TDCs required for given array size. Monte Carlo numerical simulations verify the accuracy of the proposed method.