Without additional context I would say "crowds of people" were in different places (possibly at different times), whereas a crowd of people was in one place.
In every city the Beatles played, there were crowds of people. The Beatles played in London and there was a crowd of people.
Or there could be crowds of people in the same place, but for a different reason. Say some are in town for a baseball game, some for a college graduation and more for an art exhibit.
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