It just seems like that would be a huge amount of knights. In a country of merely 700.000 people, substantially less than most european kingdoms as I understand it, that would be more than 2000 knights with castles of their own. For larger kingdoms, like France, that number becomes several thousands. Not to mention there were probably household knights who didn't own land but were servants of, say, a Baron.
All I'm thinking is that, damn, that is a LOT of nobles for a king to keep track of, isn't it?
There were not so many knights as a % of the population as you estimate. (At least in England. In parts of France the % would have been higher than it was in England at times.)
In England, at the time of the Domesday Book (1086), there were about 6,000 manors.
A manor was theoretically the amount of land and people needed to support a knight (average population of a manor estimated at 300-350 - Estimated population of England at the time of the Domesday Book, circa 2m)
Supporting a knight meant supporting the housing and food for a knight, his family, and his servants. Supporting arms and armor. Supporting a warhorse. Plus, a "knight's service" did not just include the knight himself as a single heavycavalryman. A knight turned out with himself, one or several armed and horsed squires, half a dozen or so infantrymen, and several packhorses.
Although there were 6,000 manors in England, there were only an estimated 1,000 - 2,000 knights. Some manors were held by free yeomen, who were not knights (or not 'held' by anyone, but owned by several free yeomen). Many manors were held by the church or crown, or nobles and had not been enfeoffed out to a knight, and knights often held more than one manor.
Not every knight would have a castle on his manor. This would be more common in the century after the Conquest, but thereafter, England was a relatively peaceful place, and knights were more apt to live in a manor house, rather than a castle.
In parts of France, this was somewhat different. In the Uzege, for example (The countryside within about a 30km radius from the small city of Uzes, in the South of France), there was a castle and a knight (Seigneur) in each of the 30 or so villages (and some more in the city of Uzes) by the 13th century. The villages had an average population somewhat larger than an English manor (perhaps 400 people). Add in the city of Uzes and the region may have had a population of 15,000-18,000, and supported circa 40 knights.
Knights, of course, were not nobles. The number of nobles was much smaller.
In England, in 1086, there were 10 Earls and less than 200 barons (even counting the higher clergy and the Barons of the Cinque Ports). Those were the only nobles in England other than the Royal Family.
In the region of the Uzege, there were only two 'nobles', the Lords of Uzes (generally holding the title of 'Count' during the Middle Ages) and the Bishops of Uzes (some of the knights in the surrounding villages owed their service to the bishop).
200 nobles is not that many for a king to keep track of.