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