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