This has to be one of the oddest quests I have ever done. Its not just a collect quest, its one where you get to leave the items for the next guy.
I am not sure if there are a limit to the number of spores, or a limit to the number of fully bloomed mushrooms, but I noticed a few things after a server reset.
At first the mushrooms were scattered here and there, then as those disappeared, the player placed ones started to bloom. I got to watch the 8 that I planted bloom in the exact order I planted them, with no relation to how fast I planted them. I planted them all in about 15 seconds tops, and there was a pause of about 10 minutes between number 5 and number 6 sprouting. To me it seems like there is either a minimum cap for the number of bloomed mushrooms (ie if there are not x number, the oldest spore will bloom) or a maximum cap for the number of spores (ie if there are more than x number of spores the oldest spore will bloom).
Either way the mechanics of this quest work, its a bit easier on the next guy to do this if the mushrooms are all in one central spot. From this point on I am going to plant mine in that clear grassy area in front of that ritual area, with the candles and the statue with the "laser beam eyes". So if you are looking for them in the future, try that spot, perhaps even plant your own there as well, make it easier on all of us.