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 numbe

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.