As long as the institution where you received your honorary degree recognizes the awarding institution(s) for which it has been granted, then it’s perfectly fine for someone with an honorary PhD to refer to themselves as Dr., regardless of how many courses they took or how much work they’ve put in at another institution. The main reason why honorary degrees are granted is that the recipient has made significant contributions to the field. The awarding institution recognizes this contribution and grants an honorific degree as a way of showing gratitude.