A file is a military term for a number of troops drawn up in line ahead, i.e. one behind the other in a column

Column (formation)

A military column is a formation of soldiers marching together in one or more files in which the file is significantly longer than the width of ranks in the formation...

. The number of files is the measure of the width of a formation of troops in several ranks

Rank (formation)

A Rank is a line of military personnel, drawn up in line abreast .Commonly, troops called to 'On the right, fall in!' do so by forming in line abreast, determining their initial position in relation to a marker. This may be a position on the ground or a single person placed previously to the movement...

 one behind the other.

Ancient Greek use

A file of men in the Greek phalanx was called a lochos and usually ranged from 8 to 16 men. For more information see lochos