How Many Herb Do I Know?
The first two editions of this guide contained only about 
40 herbs and so we never considered the need to limit the 
amount of herbs that a PC with Herbalism proficiency 
knew. Now there are a lot more herbs and such a rule does 
seem necessary. If PCs knowing of all these herbs does 
not cause a problem in your games, then just assume they 
do know all of them. If however, you want a rule to deal 
with situation, I would suggest the following. Once again 
I assume that you games uses a 3d6 Intelligence score 
similar to that used in D&D, AD&D, Dragon Warriors, and 
Tunnels and Trolls. 

The PC herbalist starts with knowledge of 3 x INT x Level 
of Herbalism Skill herbs. That is a PC with a herbalism 
skill of 1 and an Intelligence of 12 will know of 36 
herbs. A PC with a herbalism skill of 2 and an 
Intelligence of 9 will know of 54 herbs. Which herbs are 
known, should be decided by the GM with reference to the 
climate and terrain type in which the PC grew up.