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As a native English speaker I disagree with this pedantry. It's correctly plural.

Dictionary.com says: Usage note 2. As a collective noun, number, when preceded by a, is most often treated as a plural: A number of legislators have voiced their dissent. When preceded by the, it is usually used as a singular: The number of legislators present was small.

[semi-extrinsic]

I find it helpful in cases like this, when in doubt, to replace the phrase in question with a single word and see what makes sense then. Here:

"a growing number of traders" => "they", so "are" is correct.

"the growing number of traders" => "it", so "is" is correct.