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 when you have three of a kind comprised of three kings (of any suit).
Win consecutive all-ins.
Win a hand with Four of a Kind.
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.
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.
Win a hand with a Straight.
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.
Win a full game.
Win three straight pots.
Win a showdown where you and the opponent have same hand (pair or better) and you have a better kicker.
Win a hand with a Flush.
Check a pat hand.
Win a showdown with just a high card.
Win a hand with a Full House.