The safe drinking number? (5)

I believe the answer is:

eight

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'number?' is the definition.
(eight is a kind of number)

I cannot understand how the remainder of the clue works.

(Other definitions for eight that I've seen before include "Two fours" , "Rowing boat crew" , "Two cubed" , "A number, sounds like swallowed" , "Number for a racing boat" .)

I've seen this clue in The Globe and Mail.

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