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