This paper demonstrates the realization of high-order terahertz (THz) magnon multiplication in an antiferromagnetic orthoferrite, Sm0.4Er0.6FeO3. The authors use intense, resonant THz pulse pairs to induce collective spin dynamics and correlation, enabling the detection of nonlinear magnon interactions up to six-magnon quanta. The THz multi-dimensional coherent spectroscopy (THz-MDCS) technique is employed to achieve high-sensitivity detection of these interactions. The results show seventh-order spin-wave mixing and sixth harmonic magnon generation, which are distinct from non-resonant nonlinearities such as high harmonic generation by below-band gap electric fields. Classical and quantum spin simulations support the findings, highlighting the importance of four-fold magnetic anisotropy and Dzyaloshinskii-Moriya symmetry breaking in the magnon multiplication process. The study also reveals the potential quantum fluctuation properties inherent in nonlinear magnons, providing insights into the quantum properties of spins in THz-MDCS experiments.This paper demonstrates the realization of high-order terahertz (THz) magnon multiplication in an antiferromagnetic orthoferrite, Sm0.4Er0.6FeO3. The authors use intense, resonant THz pulse pairs to induce collective spin dynamics and correlation, enabling the detection of nonlinear magnon interactions up to six-magnon quanta. The THz multi-dimensional coherent spectroscopy (THz-MDCS) technique is employed to achieve high-sensitivity detection of these interactions. The results show seventh-order spin-wave mixing and sixth harmonic magnon generation, which are distinct from non-resonant nonlinearities such as high harmonic generation by below-band gap electric fields. Classical and quantum spin simulations support the findings, highlighting the importance of four-fold magnetic anisotropy and Dzyaloshinskii-Moriya symmetry breaking in the magnon multiplication process. The study also reveals the potential quantum fluctuation properties inherent in nonlinear magnons, providing insights into the quantum properties of spins in THz-MDCS experiments.