This is a hard to source answer.

The gist is..... you can't really put a bound on how much you don't know. Furthermore, especially when talking about "dying out", how do you differentiate subcultures from cultures from civilizations? For instance, is the Byzantine empire the same civilization as the Roman empire? How about the Roman republic? How about pagan versus christian rome? And these are all differences that are extremely well attested.

Now, we've been finding a TON of lost cities in south america recently, and I'm sure there are arguments to be made about distinct civilizations. But if you're wondering whether another Roman empire could have existed in the great plains or the steppes or Australia or whatever, there is simply no evidence for this and no rational reason to think there might have been without the aforementioned evidence; on the flip side, it's equally irrational to assume we've found "all" the civilizations.