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