A model based on disk-stability criteria to determine the number of spiral arms of a general disk galaxy with an exponential disk, a bulge and a dark halo described by a Hernquist model is presented. The multifold rotational symmetry of the spiral structure can be evaluated analytically once the structural properties of a galaxy, such as the circular speed curve, and the disk surface brightness, are known. By changing the disk mass, these models are aimed at varying the critical length scale parameter of the disk and lead to a different spiral morphology in agreement with prior models. Previous studies based on the swing amplification and disk stability have been applied to constrain the mass-to-light ratio in disk galaxies. This formalism provides an analytic expression to estimate the number of arms expected by swing amplification making its application straight-forward to large surveys. It can be applied to predict the number of arms in the Milky Way as a function of radius and to constrain the mass-to-light ratio in disk galaxies for which photometric and kinematic measurements are available, like in the DiskMass survey. Hence, the halo contribution to the total mass in the inner parts of disk galaxies can be inferred in light of the ongoing and forthcoming surveys.
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