There is not a specific size, it totally depends on the situation. Could be as few as less than a dozen to as large as several thousand. In modern times, most troops or soldiers are assigned to units (squads, platoons, companies, troops, battalions, squadrons, regiments, brigades, divisions, corps, armies, etc). Most units have specific sizes numerated for them. The sizes vary depending on the historical period of time and the nation. If you are trying to establish how many soldiers are at a specific location at a specific period of time, it is best to ask that question directly.