Here is the full list of all 20 Poker Night at the Inventory achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Win a hand with a Straight Flush.
Win a hand with Four of a Kind.
Win a hand when you have three of a kind comprised of three kings (of any suit).
Knock out Max after he's bought in with his special item.
Knock out Tycho after he's bought in with his special item.
Knock out Strong Bad after he's bought in with his special item.
Win a game on hard.
Knock out The Heavy after he's bought in with his special item.
Win consecutive all-ins.
Win a showdown with just a high card.
Win a full game.
Win three straight pots.
Win a hand with a Flush.
Win a showdown where you and the opponent have same hand (pair or better) and you have a better kicker.
Win 12 or more hands.
Check a pat hand.
Win a hand with a Straight.
Win a pot with two or more opponents when you are all in.
Win a hand with a Full House.
Win a hand with a 6 card and a 9 card of any suit.