Where does the 126 come from in the Jewish religion?

Where does the 126 come from in the Jewish religion?
Jews give gifts of money in multiples of 18 as a result. 7 x 18 = 126.
So then why is it 126? If it's multiples of 18, then why not 2 x 18 = 36 or 3 x 18 = 54? Where does the 7 come from to equal 126??
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