I read take niacin only if you have a reaction, especially when you start the B12. Niacin slows down the methylation, so only take niacin if you really need it. So if I really need it, how much niacin should I take?

Just take one capsule. So usually that would be about 500 milligrams. It depends on the supplement of course, but it's usually about 500 milligrams per capsule.  It would be even better if you could find a supplement with just 100 mg per capsule. But yeah, just take one.