Collective nouns are words that describe groups of animals, like a "flock" of birds or a "herd" of elephants. There isn't a specific word for a group of flamingos, but "flock" is often used.
It's possible you're thinking of the term "flamboyance", which describes something that is showy or extravagant. While this might be used to describe a group of flamingos due to their bright colors, it's not a proper collective noun.