I've actually had that happen quite a bit, pretty regularly, in fact. I had a build a while back where I was getting a few levels of ward early on to be strong early game w/o too much mana cost and then the rest after level 10. When I got the first level, at char. level 1, I'd just have three spirits. But if I hit 8 or 9 with only the first level still, I'd notice my army had grown to 7 or more. I think at one point I did count ten.
Of course, I don't have any screen shots, but it happened so often that I bet I could do it without much effort. And to answer Ptarth, I was playing (back then) w/o mods, I was able to lose and regain them, there were other oaks occasionally but the spirits were clearly mine, and they seemed to build up over time.