Herb Patch Gathering

Is the success rate fixed at 100% or is it determined by your rank of Herbalism? Also, is it safe to assume you can get lucky and get multiple herbs in one gathering like you can with homestead stones and non-homestead gathering?

You can get lucky and gather multiple herbs at once.

I'm fairly sure the success rate is the same as the equivalent herb patches in dungeons. My elf with rank F herbalism fails two out of three tries to pick base herbs, while my human with rank 9 or 8 or so fails once every five tries at most. I haven't tried picking more difficult herbs with my elf yet, but I imagine it'll also follow the pattern.

Herbalism affects how fast you can actually gather the herbs. As a result, in places with a fixed gathering rate which isn't affected by herbalism, the picking will take quite a bit longer; for example, I can clear a base herb patch out in the field in the time it takes to gather two or three herbs from an herb pet, assuming all successes on both.