How many milligrams of adderall(amphetamine salts) does it take to possibly cause overdose?

That depends on many: Factors, including age, body size and the presence or absence of other health issues. There have been reported deaths in children with doses as low as 2 mg, but the ld50 is in the range of 1.5 mg/kg or so. In a 70 kg person, this would be about 120 mg as a dose that would be toxic to half of those who ingested it. People with underlying heart and blood vessel diseases may be more sensitive.