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