A generic theory of skyrmion crystal (SkX) formation in chiral magnetic films is presented. We numerically demonstrate that a chiral film can have many metastable states with an arbitrary number of skyrmions up to a maximal value. A perpendicular magnetic field plays a crucial role in SkX formation. The energy of a film increases monotonically with skyrmion number at zero field while the film with $Q_m$ skyrmions has the lowest energy in a magnetic field. $Q_m$ first increases with the magnetic field up to an optimal value and then decreases with the field. Outside of a field window, helical states of low skyrmion number densities are thermal equilibrium phases while an SkX is metastable. Within the field window, SkXs are the thermal equilibrium states below the Curie temperature. However, the time to reach the thermal equilibrium SkX states from a helical state would be too long at a low temperature. This causes a widely spread false belief that SkXs are metastable and helical states are thermal equilibrium phase at low temperature and at the optimal field. Our findings explain well the critical role of a field in SkX formation and fascinating thermodynamic behaviours of helical states and SkXs. Our theory opens a new avenue for SkX manipulation and skyrmion-based applications.
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