Hi Dagma,
I suspect the reason your therapist said it doesn't matter how much B-12 you take is because it is a very safe vitamin -- there is no known upper dose/limit. Some people take as much as 5 mg/day without any problems. Folks here recommend the type called
methylcobalamin, as it is better absorbed than other forms. Any dosage between 1 mg - 5 mg (1000 mcg - 5000 mcg) should be ok.
Here is MrsD's B12 thread.
A good multi and a good B complex will just make sure you're getting what you need in case you're not getting them in your diet.
Here is
MrsD's post on magnesium supplements.
Doc