From Halopedia, the Halo wiki
A division is composed of three regiments or brigades, as well as a "Division Headquarters Section", ideally totaling to approximately 14,000 personnel. The additional personnel in the force number given are support units, including medical, combat engineering, maintenance, and military police. Divisions are typically commanded by a brigadier general or, occasionally, a major general. As part of their command career path, master sergeants have administrative responsibilities within a division, while sergeant majors serve as assistant commanders within a division. The UNSC Army fields airborne, armored, and infantry divisions, while the UNSC Marine Corps fields marine and reconnaissance divisions.
Judging from a comment from Fleet Admiral Lord Terrence Hood, there are no less than four divisions of Orbital Drop Shock Troopers in existence, including the 65th and the 105th Shock Troops Divisions.