Oddly I found this after engaging with a new client that is running into major issues with route limitations in the mx platforms trying to do my own research on this. They have over a thousand retail locations (and growing), and found pretty quickly with some 15 subnets/routes per location, things started blowing up at a certain number of deployed sites. It apparently took a coming to jesus meeting with Cisco to get them to admit to route limits in the platform, some 12000 prefix limit it seems, and as you said isn't documented anywhere I can find either. Now they're having to introduce summarization, which as an old organization wasn't always built with such in mind, and proving a challenge NOT to blackhole traffic when doing so. Customer is not at all happy, bringing in consulting (ie. me) to see what can be done, outside gutting them. I'm trying to see if this is a global platform limit across hardware, or whatever SE/Var spec'd the hardware just totally screwed up in not knowing this going too small of boxes. I would have some conversation with your AM/SE about this, seems I'll be having some with theirs.