I hate to disappoint you, but we really have no way of knowing. Early 11th century French sources are sparse at best, and they tend overwhelmingly to focus on religious matters, kings, and great lords, in descending order of priority. The kind of organizational and economic details you are interested in - in this case, the manpower available to the lower nobility - are irretrievably lost to us because no one thought it worth writing down. To the extent that these people are even mentioned in chronicles or charters, it's usually in passing, and common soldiers escape notice almost entirely.

Even if we had this kind of information, though, it's not as if we could plug the number of knightly vassals into an equation which would give us total manpower. To misquote Cicero, the sinews of war are money, or at least wealth, and this was as true in the medieval period as it was in republican Rome. Aristocrats raised manpower in a variety of ways, not only through service-for-land agreements, and two lords with fifty infeudated knights could differ considerably in wealth and power.

For what it's worth, William of Normandy invaded England in 1066 with perhaps 8,000 men, of whom about 2,000 were knights. But these were not all his vassals: many were adventurers or mercenaries from Brittany and Flanders, and others belonged to the ducal household. We know even less about the 6,000 or so foot soldiers in William's army. It's likely that at least some were hired men, but even that is going out on a ledge.