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