We report on long-range spin wave (SW) propagation in nanometer-thick yttrium iron garnet (YIG) film with an ultralow Gilbert damping. The knowledge of a wavenumber value is essential for designing SW devices. Although determining the wavenumber in experiments like Brillouin light scattering spectroscopy is straightforward, quantifying the wavenumber in all-electrical experiments has not been widely commented upon so far. We analyze magnetostatic spin wave (SW) propagation in YIG films in order to determine the SW wavenumber excited by the coplanar waveguide. We show that it is crucial to consider the influence of magnetic anisotropy fields present in YIG thin films for precise determination of SW wavenumber. With the proposed methods we find that experimentally derived values of are in perfect agreement with that obtained from electromagnetic simulation only if anisotropy fields are included.