Two, but those are just the enantiomers dextroamphetamine and levoamphetamine. Neither has any atoms extra or missing. It looks like four amphetamines because those two cationic isomers are in salts with two different anions.

Each enantiomer exhibits different effects so I don't know if that is relevant. (Even if you count dextroamphetamine sulfate and amphetamine sulfate as one, I'd still probably call it 3 because dextroamphetamine saccharate and amphetamine asparate monohydrate (the other two parts of adderall) do indeed have different forumlas).