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