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