Plenty of functional Adderall users also, which is mixed amphetamine salts.

And although more rare, functional desoxyn users too, which is a methamphetamine pill typically used for narcolepsy and severe obesity.

How many functional Ritalin/Adderall/Desoxyn users are injecting?

Probably as many as functional Tylenol users are injecting. In my meth days I knew of 2 people that injected. It was always smoked or, less commonly, taken orally.