The originaldefinition of "twank" is a term created by old school skater kids to describe a twisted or busted up ankle. Often used along side the word "dank", which is similar to twank but describing the upper body parts.
-Today I went skating, I twanked my ankle up real bad, and danked my right wrist when I ate shit.
Can be used for the word two, twenty or twelve, but more often when describing a twelve pack of beer, or a 20 dollar bill. You can modify the word twank to encompass a 24 pack by making it twank fo'. You can double up the word for added effect.
for a twelve:
Joe: Hey man, how much beer are we getting?
Bruce: Dude, we're pounding a twank tonite!