From Halopedia, the Halo wiki
A company contains four platoons plus a group of personnel assigned as "Company Headquarters Section", which totals to 164 personnel. A company is commanded by a captain, with a first lieutenant serving as the company's executive officer. In addition, a company may appoint a "company sergeant" - a staff sergeant or gunnery sergeant who acts as the assistant company commander. The twelve companies of a brigade or regiment are identified phonetically "A" (Alpha) through "M" (Mike) with the exclusion of "J" (Juliet). The first battalion consists of companies A through D, while the second battalion is composed of companies E through H, and so forth.
Generally, an infantry company would be composed of three infantry platoons and one heavy weapons platoon. Though, these numbers are flexible depending on the unit type and mission. "T" companies, for example, eschew this model and mix their heavy weapons personnel into each platoon so all the platoons have one heavy weapon squad. An ODST company may be comprised of between six and eight combat teams (though this can vary from company to company), with each combat team comprised of 10-14 personnel, allowing ODST companies to have roughly between 60-112 personnel on average.
List of known companies
List of appearances