I feel your pain and frustration OP, I really do. I too have sunk a lot of time into searching for these aaaand iv not found a single one.
Iv been to one after finding co-ordinates on the forum, but iv never found one myself. To me this is somewhat broken. If there is a technical limit, is that inclusive of everything on a planet? I am pretty sure iv seen planets with 4 - 5 persistent bases on it.
In my mind, if a planet has volcanism we have lots of obvious objects where to find it.
-Mountain ranges
-Big impact craters
-Canyons
-Stress locations (such as the terminator in tidal locked systems)
Thus far I dont think anyone has reported back finding any volcanic peaks, any within large impact crators or obvious stress locations. Thus far i believe it is mostly been Cracks, Canyons and occasionally planes near the canyons
Id love to hear back from FDev regarding this and the possibility of improving the feature. I mean, in our own solar system we can see these objects from far away in some cases, we can see bases on the surface in orbital cruse...
It just leaves me to speculate why and probably get it personally wrong, but seems like based upon the planetary generation the automatic selection and placement of a vocalic cluster could be the issue. That the client cant easily figure out what are the tectonic regions based upon the seed it gets from the servers to generate the surface. So generating it is perfectly ok, but the locations are being retroactively selected rather than it being a core part of the seed. This means the limit being at 2-3 is more about limiting database size than anything else.
I dunno - wild speculation from me at this point to be fair.
Said it before and Ill say it again... this has been the first feature added to ED that Iv been quite throughly disappointed with, I had a vision of driving into a foggy basin and within that fog there being volcanic activity, that you would see these things in some of the more hellish places quite prominently. Right now... its easier to find a crashed wreckage of an F63 or Diamondback on a planet than a natural feature such as volcanism...

sad