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 Max 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 where you and the opponent have same hand (pair or better) and you have a better kicker.
Win three straight pots.
Win a hand with a Straight.
Win a pot with two or more opponents when you are all in.
Win 12 or more hands.
Win a full game.
Win a hand with a Flush.
Win a hand with a 6 card and a 9 card of any suit.
Check a pat hand.
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.
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.
Win a hand when you have three of a kind comprised of three kings (of any suit).