I found following sentence from www.bbc.com. In actual fact, I seldom, if ever, find "there are number of" in articles as majority of writers often use "there are a number of" or "there is a number of". Furthermore, because this sentence is from bcc.com, I don't think this is a mistake. Therefore, I would be really grateful if someone could help me to distinguish the difference between "a number of" and "number of".
There are number of potential roles but the most obvious is as a "selfie" inspection tool.
Reference :-
There are number of potential roles but the most obvious is as a "selfie" inspection tool. It's a mistake. Clive
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There are number of potential roles but the most obvious is as a "selfie" inspection tool.
It's a mistake.
Clive