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