A very rare few at any one time are serving as Liasion Officers or Exchange Duty Officers at any one time with the Royal Marines, perhaps a dozen at any one time on a routine basis.
The number of Marines at the Battle of New Orleans was 58. Marine corps historians claim the Marines anchored the center of Andrew Jackson's line. The Marines were on the right side of Jackson's line...
That can depends on what size of a unit you are talking about. A squad usually runs about 10 people. A platoon will have 3 to 5 squads. A company has two platoons. And a battalion normally has 4...