I would imagine that its a hard coded limit. I don't think anyone has gotten anywhere near this number considering that this is the first that I am hearing of such a large deployment consideration.
Really don't know how well this is going to work. These really are unchartered waters. And considering that when you make changes to PPTP it disconnects the client(s), I wouldn't see any difference on the PPPoE side as well. If you are serious about this large of a deployment I would definitely do some type of test before going live.