1 HF is sufficient for all NAT operations, however it must be working at the time of entry.
Being equipped with a working sat phone installation, together with the relevant numbers (care, some are inmarsat short codes, those won't work on iridium) is very helpful.
In case of HF failure please revert to satcom only in case of urgency, they don't really like to get called. Just for position reports relay them via an airliner on 123.45.
We're there somewhere, bored, happy to help you.