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