A platoon was a group of soldiers typically thirty to fifty strong. It was the smallest military unit led by an officer.
At least one Starfleet platoon was involved in fighting the Klingon attack on Ajilon Prime in 2373. (DS9: "Nor the Battle to the Strong")
In 2374, the Starfleet survivors of a captured Jem'Hadar fighter which crashed on an uninhabited class M planet encountered Jem'Hadar soldiers already present that were members of a platoon led by their Vorta master, Keevan. (DS9: "Rocks and Shoals")
Also that year, Yelgrun brought an entire platoon of Jem'Hadar to Empok Nor during a prisoner exchange between the Dominion and the Ferengi. (DS9: "The Magnificent Ferengi")