Every branch of the military, including ONI, have squads consisting of anywhere from a few to dozens of members.
Spartans, while able to operate effectively individually, are normally grouped into fireteams consisting of between three to six members. These Spartan fireteams are the ultimate weapon for pinpoint force application in combat and non-combat scenarios.
OFFICE OF NAVAL INTELLIGENCE
ONI has units everywhere, some out in the public, but most operate in the shadows, affecting the galaxy around them without anyone ever knowing they were there.