"flights" is a plural form, and "there were" is at the front, so shouldn't it be without "an"? To be accurate, shouldn't it be "there were estimated 45million flights"?
Great question! "An" is considered an indefinite article. Indefinite articles are used to indicate that a noun refers to a general idea rather than a particular thing. This sentence is saying that they don't know the exact number of flights worldwide, which is why the narrator uses "an estimated 48 million flights." Saying "there were estimated 48 million flights worldwide" is not a complete sentence. To a native speaker, this sentence does not sound correct. Here are a few examples with the indefinite article "a." Each sentence refers to a general idea rather than something specific. Ex: Could you please give me a book? Ex: I need an honest opinion. Ex: I need an umbrella.