The same no. of troops that are in Afghanistan now will do. When I said hatred gainst American troops I meant that they hate American soldiers on their soil as they see them as invaders( same goes for all foreign troops in Afghanistan).Asuuming there is the political will for such a course of action, who will fund the operation and who will contribute the troops? And for how long will ''UN and military border guards'' be expected to ''prevent the states from going to war'' and under what mandate?
On the contrary, I'm very aware, though not familiar, with the Pashtun code of Pashtunwali :.
I think you're mistaken, it's not the ''common hatred against American troops'' as you put it, but rather the dislike, in common with most people, of having foreign troops on one's soil. To say that 'hatred' of American troops unites the Afghans is too simplistic.
To create several new states based on enthnicity would create a very dangerous precedent, apart from being in this day and age morally wrong. This would lead to the Pashtuns in the North West Frontier, the Baluchis in Baluchistan, the Tajiks in Smarkand and Bakhara, etc, the Kurds in Turkey, Iraq and Syria, the Turkmons in Iraq etc, to seek independent states.
As for other ethnicities demanding their own state, I am pretty sure the Turks and Iraqis are more than capable of keeping them in check, they have done so for quite some time.
Heavy military presence at the borders will be needed for some time 7-10 years after that the UN peace corps can take over. Sure this might seem a long time but in my view its the best option. A somalia like situation in a country that is so strategically located and so near to key US allies, like Afghanistan is not viable.
You are right though having so many different states will be problematic, a more realistic approach would be create 3-4 states for 3-4 of the biggest tribes there.
Lastly do you really believe that the quagmire the US is stuck in will ever end if Afghanistan is to remain whole?