My dr told me to take folic acid about 6 weeks at least before trying to conceive. how many milligrams should i take?

At least 400 mcgs: The recommended daily amount of Folic Acid is 400 micrograms (which is 0.4 mg) but many prenatal vitamins contain 800 mcg and since Folic Acid is harmless it may be reasonable to err on the side of a higher dose. Many people do not convert Folic Acid to the active form, 5-methyltetrahydrofolate (5-mthf) so to be totally safe you might look for a supplement that contains this most active form.

1-5 mg daily orally.: I would go even further and recommend that you take folate (folic acid) 1-5 mg daily throughout your reproductive life. Women taking Folic Acid for at least 3-12 months prior to conception have decreased risk for preterm birth fetal cardiac and brain anomalies!