A platoon (in the US Army) is the smallest military unit led by an officer. Two or more platoons make a company; a company is commanded by a captain.

In a British infantry battalion there are 3 rifle companies, 1 support company, 1 HQ company. Each rifle company is split into 3 platoons, and a HQ element. Each platoon is then split into 3 Sections of 8 men. Thee platoon is led by the platoon commander which is normally a 2nd lieutenant or lieutenant and a platoon sergeant. The sections are normally led by a corporal with a lance corporal as his second in command or 2 ic

Do you mean in Headquarters company or 3rd platoon in some other company? Actually it probably doesn't matter. No master list is maintained of any company, let a lone a platoon.

The answer depends on the Country and the Period and even the type of unit. For example a Paratrooper platoon is smaller than an Infantry Platoon. For WW2 US infantry Division, the Platoon consisted of 3 Squads of 9 to 12 men per squad.

"Platoon" is capitalized when it is used as part of a proper noun, such as the title of a specific military unit like "Alpha Company, 1st Platoon." Otherwise, it is not capitalized when used generically to refer to a group of soldiers or a military sub-unit.