Typically, a Division is composed of three Regiments which, in addition to a "Regimental Headquarters Section", encompasses 14,000 personnel in total. The extra personnel include medics, combat engineers, military police and logistical units. A Division is commanded by either a Brigadier General or Major General.
Judging from a comment from Lord Hood, there are no less than five divisions of special forces ODSTs in existence, which likely puts the total number of UNSC Marine Corps divisions much higher. It is unknown as to how many Divisions the UNSC Army is comprised of.