Spartans are generally deployed in smaller teams like you saw with Halo: Reach and Spartan Ops. Very, very rarely will you see them in large numbers that would see them organized them into platoons, companies, and even a small battalion. Such occasions include Operations PROMETHEUS, TORPEDO, and RED FLAG, as well as the contingent aboard UNSC Infinity. Keep in mind that these missions constitute very high-profile taskings like destroying Covenant infrastructure and command. Like most special operations units today, these soldiers may be organized and have the numbers to comprise a brigade-sized unit, but very rarely are they ever deployed as a whole.

Meanwhile, the Orbital Drop Shock Troopers can be considered more conventionally focused. They're the elite shock troops of a Marine Corps of many, many millions. It was noted that as late as September 2552, there were at least three divisions (10,000-20,000) of these Marines. During the Fall of Reach they had a whole regiment in a single territory. Unlike the Spartans, as we saw in things like Halo 3: ODST, they are routinely deployed at platoon, company, and battalion-strength for massed orbital drops akin to the U.S. 75th Ranger Regiment. However, like in that very game, they can also be detached into smaller fireteams in the special reconnaissance role.

Greater numbers and capability creates a more versatile force with the ODST's when you consider that they likely have organic combat support assets such as field artillery, heavy mortars, combat engineers, air defense, and logistics like the airborne forces of today. Spartans, among other special reconnaissance units, usually have that support provided by whatever forces are in their area of operations. The larger ODST's have greater autonomy in that regard.

There will always be a need for Helljumpers and Spartans just as we have the differences between Delta Force, the Green Berets, and Rangers.