...vary greatly depending on the branch of service, mission and other factors. Infantry platoons usually number around 40 soldiers while supporting platoons (mortar, recon, anti-armor, etc.) can be as few as 10 or as large as 50+.
Technical platoons such as communications, intelligence, maintenance, etc can also vary greatly depending on whether they're direct, intermediate or higher level support. The Airframe and Powerplant platoon in the aviation maintenance company I was assigned to at FT Campbell had around 60 soldiers at full strength while the Armament platoon I was in had 30.