We present the results on the relativistic six-pion scattering amplitude at low energy, calculated at $\mathcal{O}(p^4)$ within the framework of the massive O($N$) nonlinear sigma model extended to the next-to-leading order in the chiral counting. For $N=3$, this approach corresponds to the two(-quark)-flavor Chiral Perturbation Theory. We also present the expressions for the pion mass, pion decay constant and the four-pion amplitude in the case of $N$ (meson) flavors at $\mathcal{O}(p^4)$.