...well that's gonna take a fuckin while, innit?
There are some pretty big patches around the map, like some huge valerian patches in the woods where The Prey quest takes place. I believe I got around 100 each of dandelions, nettle, and thistle for the Weeds quest and I only picked weeds in the monastery's three flower beds (which was WRONG apparently, only supposed to do two and not three, how the hell are you gonna fail me for that?!), there's a whole rest of the yard to do. There is a counter in statistics for herbs collected, and yes, that counter needs to hit 10k for it to trigger.