In 1086 AD, I'm getting estimates of England having ~2 Million people. At a rate of 0.5% magic users, you'd have ~10,000 mages in England in that time. ~24% of them would be <20 yrs old, though. So we'd have 7,600 mages "of age". A further 6-12%(and arguably, the eldest of the 24%) would be in mage academies at best, or worst, nothing notably more than a mundane peasent.
That leaves ~6,400 mages of "trained age" in the Kingdom, although I've not yet culled this number for lack of raw skill worth training, yet.