How much dexedrine (dextroamphetamine) would it take to get messed up?

Depends: That depends on what you mean by "messed up". If you are referring to a euphoric feeling (ten feet tall and bullet proof) it would vary with individual patient dosing, tolerance to the medication, pharmacological interaction etc. I must caution you that doses of these medications that produce euphoria can also produce end organ damage (heart , kidney, brain).