At first glance, it might seem that the English army in during the Agincourt Campaign is quite well documented. The Soldier in later Medieval England has 11,285 records for 1415.
But. In theory, every man at arms that fought at Agincourt should be listed three times: in a pre-departure muster, a post campaign accounting, and the Agincourt Roll copied by heralds in the late 16th and early 17th century. Since the herald weren't interested in common archers, the number per retinue is given but not their names in the Agincourt roll, so archers at the battle should be listed twice. Men invalided home should be listed both in the initial muster and a muster of invalids.
In practice, it is possible for the same man to be listed more often: some post campaign accounting also listed men who died at Harfleur or were sent home as invalids, distinguished in the list but not in the database They could have been on an invalid list as well. In addition, the Agincourt Roll double counts at least one retinue, that of Sir Henry Huse, both as part of a larger retinue and as an independent listing. It also lists the names of some men who died or were still sick at Harfleur, had been sent home, or were killed before the battle.
So there are less than half as many records as we would expect if all the documentation had survived. The Agincourt Roll is clearly incomplete: it conspicuously omits the Duke of York, who died in the battle, and his retinue, and only accounts for 2,496 archers in an army that at the lowest estimate had twice as many.