Can collective nouns be plural? For example, can I say armies instead of army if I'm talking about different types of armies, or crowds. crowd and army are already plural but can I add an s after them if I'm talking about different armies and crowds?
A collective noun indicates a collection or group, but where there are several different groups, a plural form may be used.
- France deployed seven armies on its borders at the start of World War One.
- There are three prides of lions in this nature reserve.
- There were once four different herds of buffalo in this area.
- Crowds are prone to mob violence.
- Nations have no friends, only interests.
- Let the nations rejoice! (Meaning all the people of the world.)
- In this period several peoples migrated out of Asia.
- One of the earliest works of paleontology was a study of The Fossil Fishes.
Do not confuse collective nouns (also called group nouns) with mass nouns (also called uncountable or noncount nouns). Some collective nouns are mass nouns, but not all. These are different qualities with different implications.
When it comes to pluralization, it does not matter whether a noun is collective or not. It only matters whether it is countable or not.
Collective nouns represent a group of individual items. In most cases, the groups themselves are also countable, and pluralize normally when there are multiple groups: gaggles, herds, parties, teams, families, and so forth.
In other cases, however, the collection is treated as if its components are undifferentiated, as if it were a substance like steel or butter. Collective nouns like coinage, personnel, or foliage are also mass nouns, and mass nouns are not normally pluralized or enumerated. Luggage is more like sand than flock. You can walk somewhere with two flocks of sheep, but you cannot bring two luggages of bags with you, any more than you can picnic on two sands.
Army and crowd are collective nouns, but not mass nouns, and it is entirely unexceptional to speak of armies and crowds. Yes, this is another of English's many lists of exceptions upon exceptions, but I would say that collective nouns which are also mass nouns are relatively few in number. I've already named the more common ones, plus staff, signage, baggage, and a few other -age words.
What you may be thinking of is the use of a plural verb with a collective noun like army or crowd, done when one emphasizes the individuals in the group as opposed to the group itself: the team are taking the field. This is far more common in British English than in American, to the point where nowadays, many Americans will consider that sentence to be ungrammatical. But that is about pluralization of the verb, not of the noun itself.