I'm not as optimistic as S'mon over this, but I don't think his numbers are without merit.
A battle with that level of actual mortality, however, would be extraordinarilly devastating by medieval standards. A loss of that sort is going to not only be legendary it's going to cause some serious disruption in terms of people's confidence in the polity.
On that level:
-Mercenaries are going to be exactly what you want tactically in that they are going to be effecient and conservative if not terribly effective or a good means of projecting power.
-Mercenaries are going to be tremendously disruptive socially. On a minor level they are going to bring a lot of military power into urban areas where it was probably previosly in rural areas. On a major level if I'm a mercenary in that sort of situation I am not going to behave. This is exactly the sort of situation that results in new dynasties or coups.
-The nobility is going to replace itself from the outside among other places. This is even true internally as widowed ladies and bastards become more important social forces. But the real issue is a host of related families from outside the borders who are going to either claim inheritance or simply show up. On the plus side immigration is probably going to take of any low level labor shortages you have. Some villages may dissapear in the interim but the disruption to your tax, legal, and educational structures is a much bigger issue.
-Your society is going to become way more stratified. If you used to have a strong rural nobility I don't think they're going to come back in the same strength. Strong nobles who survive are going to become a lot stronger as they goble up undefended local holdings. Peasant rights and royal rights are going to go to
unless they make the right alliances or provide the right sort of incentives for negotiation. There may also be an increase in the middle class but if your society had a fairly flat structure before with prosperous lower classes and slightly less wealthy but broad upper classes that's going away.
-The overall military make up of your society is probably not going to come back. If the nation has similar neighbors they can import military tech and knowledge from they're going to recover their tech (in the soft sense) faster, but if you had anything unique or you've got problems with distributing imports then your next war is going to be fought differently. At the least a very different sort of people will be hiring. This isn't necessarilly bad, if you have any native or unique modes of warfare you haven't used much before now is the time to develop them.
One factor I'm unclear about:
Were the 5000 permanent troops the levied nobility or an extraordinarilly large royal guard?
Cause if I'm one of the remaining or incoming nobility and the central court lost an asset like that I'm seriously going to consider renegotiating my relationship with that court, and if I'm that court I'm going to have seriously consider the expediency of such an expensive asset as well when the levies need so much shoring up.
What's the local geopolitical situation like? How fast is that horde going to replace itself?
