Is this possible? I always thought herbs were one time pickups in those segments. The mobs have a random chance to drop ammo and certain segments seem to have endless spawning mobs.
If an item does not appear in a crate, but appears on top of a random box or on a shelf, then it's more often than not always going to be there when you go through the scenario again. This is particularly true for Herbs and First Aid Sprays.
To farm a segment for health/ammo, just finish the segment of the chapter, look for the Saving icon to appear at the start of the next segment, and then exit out. Rinse, repeat.
The best farming points each chapter are, as done in Amateur:
Jake/Sherry
Chapter 1-3: Underground Passage
Not much ammo in here, but the segment is 30 seconds long and always guarantees a a few thousand skill points, 1 Green Herb and 1 Red Herb.
Chris/Piers
Chapter 3-1: Bosawan Entrance
Just a few rooms long, but there's a First Aid Spray in the last room before the next segment, and a Green Herb in the outside area after going through the gate, but before entering the building.
Chapter 3-2: Bosawan Courtyard
Another great area with tons of Grenade Launcher ammo, a First Aid Spray and almost always, two Green Herbs among the crates in the lower area.
Leon/Helena
Chapter 3-1: Airplane
It starts off with a boss fight, but there's always Magnum Ammo in the cockpit. There are 4 Green Herbs scattered throughout the plane, as well as a First Aid Spray in the cargo bay.