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 12 or more hands.
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 The Heavy after he's bought in with his special item.
Check a pat hand.
Win a hand with a Full House.
Win a showdown where you and the opponent have same hand (pair or better) and you have a better kicker.
Win a pot with two or more opponents when you are all in.
Win a hand with a Flush.
Win a showdown with just a high card.
Win a full game.
Win a hand with a Straight.
Win three straight pots.
Win a hand with a 6 card and a 9 card of any suit.
Knock out Tycho after he's bought in with his special item.
Win a game on hard.
Win consecutive all-ins.
Knock out Strong Bad after he's bought in with his special item.
Win a hand with a Straight Flush.
Win a hand with Four of a Kind.