Here's why:
* Countable noun: When referring to a group of soldiers, "troop" is countable. You can have one troop, two troops, etc. For example: "The troop marched across the field."
* Collective noun: When referring to a group of people or animals, "troop" is a collective noun. It represents the group as a whole. For example: "A troop of monkeys swung through the trees."
The specific meaning of "troop" will depend on the context of the sentence.