The collective noun used to describe 'a number of soldiers' is:

Collective Noun:
A Collective Noun is the name of a number (or collection) of persons or things taken together and spoken of as one whole.
Ex:
Crowd, mob, team, flock, herd, army, fleet, jury, family, nation parliament committee etc.

Few things to be noted:
A fleet = a collection of ships or vessels.
An army = a collection soldiers.
A crowd = a collection of people.
- All of these are used as Collective Noun.

[Source: High School English Grammar and Composition BY Wren and Martin.]