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