I challenged myself to collect every single Nirnroot in Oblivion once... Not sure if I did it, I got about 11 but there was prob alot more.
There's definitely more, considering that you need at least ten just to make the most basic version of the potion you get for that quest...
Really, though, it seems to me that collecting them all really isn't that rewarding in Oblivion, unless you just do it for the sake of completion.
In the screenshot, though, it does look like it may be a Nirnroot, if it weren't near water, I might have thought it was something else, but the fact that there happens to be a plant that resembles Nirnroot right next to a body of water seems too much of a coincedence for it to not be a Nirnroot. And if it is indeed a Nirnroot, than I only hope that in Skyrim, if there's going to be a quest surrounding them, the rewards are better than for Seeking Your Roots. While that kind of quest tends to be annoying to begin with, I at least want a better reward for going around the world to collect those plants than a potion I'll probably never use.