The number of legions depends on the period. Some legions where destroyed in battle, example: Legio XVII, Legio XVIII and Legio XIX where destroyed in the Teutoburg forest and the Aquila* standards...
That really depended upon the general. Roman generals recruited their own armies so there would be as many legions as; first, the general could afford and second, as many as he could convince to join...
The number of legions depends on the period. Some legions where destroyed in battle, example: Legio XVII, Legio XVIII and Legio XIX where destroyed in the Teutoburg forest and the Aquila* standards...