Vit B6 shouldn't be taken in excess of 25 mg per day. Very high dosages (600 mg/day) suppress prolactin secretion and therefore will reduce milk production. Excessive oral doses in infants have been reported to produce side-effects in infants (sedation, hypotonia and respiratory distress), but no concerns have been reported via breastmilk.
The vitamin B6 content of milk is directly related to maternal intake, but vitamin B-6 deficiency is rare in the United States. According to one study, B6 levels in women who supplemented with 2.5 mg per day are twice as high as those of unsupplemented women. As far as I could see, this reference didn't discuss moms supplementing with more than 10 mg per day (they were concerned more with deficiencies).