You can gain the entire Healing Herbs feature, encompassing all the listed herbs, as one discovery.
The text of the Master Healer's 3rd level feature Healing Lores explains this:
...at 3rd level, you make a discovery in your quest to understand the mind and body... Choose from the following discoveries.
The text then continues to list Healing Herbs, Friend to All, Softer Underneath, and so on, each with their own heading. Each is therefore a distinct discovery.
Note that within the Healing Herbs discovery, the following text shows that the discovery encompasses any medicinal herb that you use in general, without any restrictive rules given. There is no indication that you must select a specific herb as your discovery.
When using medicinal herbs, use these effects instead of the normal effects of the herb.
This is followed by a list of herbs including athelas, hagweed, and so on, indicating how your enhanced use of medicinal herbs in general applies to each indicated herb.
There are 6 discoveries to choose from.
Since Healing Herbs is on equal footing with the rest of the listed discoveries as a full-fledged discovery option, you can pick from the following 6 options whenever you learn a new one:
- Healing Herbs
- Staunching Song
- Friend to All
- Softer Underneath
- Relief from Long Burdens
- Weapons of the Enemy