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