Eight, as it turns out, according to CBS News Crimesider. Babies, notable lightweights, don't know the meaning of moderation—the tot in question had ramped up to a Rip Torn-esque .33 blood alcohol level by the time his grandmother arrived to pick him up.
On a related note, a classic Yahoo! Answers interaction:
"Ok [sic] to rub whiskey on teething babys [sic] gums when alcoholism runs in family?"
Answerer: Give him the bottle
Thanks, the Internet.