Lately I've been ordering 2 at a time, targeting to do two orders about 2-3 days apart, 3x a month. Yes, that's over $2K. I haven't had any problem getting miles credited. Even though it was too soon for my regular cycle, I ordered 2 cards tonight.
Based on my personal experience, I would strenously suggest avoiding more than two transactions in every 2-3 days. You never know what will trigger a declination. (And unfortunately, BHN has no idea either, so they can't help you or give an intelligible reason.)