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