It actually depends on the configuration of your AK, but the rule is no more than 10 parts can be imported. The ATF determined the AK can be as many as 16 parts, so you pick and choose as long as you don't have more than 10 imported parts:
Receiver
Barrel
Trunnion
Muzzle attachments
Bolt
Bolt carrier
Gas piston
Trigger
Hammer
DisconnectorButtstock
Pistol grip
Forearm handguards (two handguards count as one item)
Magazine body
Magazine follower
Magazine floorplate
So, for example, if your AK doesn't have a threaded barrel, that's one less part that has to be changed out. Equally, if the muzzle attachment is permanently attached to the barrel, it counts as part of the barrel and not an extra part.