We derive the upper limit to the ejecta mass of S190814bv, a black hole-neutron star (BH-NS) merger candidate, through radiative transfer simulations for kilonovae with realistic ejecta density profiles, as well as detailed opacity and heating rate models. The limits to the ejecta mass strongly depend on the viewing angle. For the face-on observations (≤45°), the total ejecta mass should be smaller than 0.1 M o for the average distance of S190814bv (D = 267 Mpc), while a larger mass is allowed for the edge-on observations. We also derive the conservative upper limits of the dynamical ejecta mass to be 0.02, 0.03, and 0.05 M o for the viewing angles ≤20°, ≤50°, and ≤90°, respectively. We show that an iz-band observation deeper than 22 mag within 2 days after the gravitational-wave trigger is crucial to detect a kilonova with a total ejecta mass of 0.06 M o at a distance of D = 300 Mpc. We also show that a strong constraint on the NS mass-radius relation can be obtained if future observations put an upper limit of 0.03 M o on the dynamical ejecta mass for a BH-NS event with a chirp mass smaller than ≲3 M o and effective spin larger than ⪆0.5.