Just add it in Config.prc and raise the value from the default. I’d be curious if the limit does appear to be raised if you double the value.
Panda3D should (in theory) automatically clean up the model pool (ie. in-memory cache) whenever it threatens to run out of memory, but I’m unsure of the precise mechanics of that. You can also periodically call ModelPool.garbageCollect() to clear out unused models.