The calcium intercalated graphite is considered to be a representative material of the graphite intercalated superconductors, which exhibits sizable anisotropy of the Fermi surface (FS). Herein, the influence of the FS anisotropy on the critical magnetic field in superconductor is comprehensively analyzed within the Migdal–Eliashberg (M–E) formalism. To precisely account for the mentioned anisotropy effects, the analysis is conducted in the framework of the six-band approximation, hitherto not employed for calculations of the function in material. For convenience, the obtained results are compared with the available one- and three-band estimates reported by using the M–E theory. A notable signature of the increased number of bands is observed for the temperature-dependent functions. In particular, the function decreases at T = 0 K as the number of the considered bands is higher. Moreover, it is argued that the six-band formalism yields the most physically relevant shape of the function among all considered approximations. Therefore, the six-band model introduces not only quantitative but also qualitative changes to the results, in comparison to the other discussed FS approximations. This observation supports postulate that the six-band model constitutes minimal structure for FS of , but also magnifies importance of anisotropy effects for the critical magnetic field calculations.