Anne Curry has a new revisionist take on the battle, saying the French army was much, much smaller than most historians have believed. Also, that the English army was a bit larger. The change in numbers for the English isn’t as large, so I’ll leave that part of it aside for simplicity.
Here is a forum post covering a lot of her argument.
I am not convinced by her arguments for the French numbers. Let’s start with Berry Herald, who she considers a significant and useful source for the battle, and I think she’s right. Berry was writing years after the battle, and we have no reason to think he was present, but he provides a detailed order of battle of the men-at-arms assigned to a dozen different commanders. 1,200 men at arms are assigned to the mounted wings, 4,800 to the vanguard, and about 3,000 to the main battle. He says the total was 10,000, so that leaves 1,000 for the rear battle.
Most of the sources agree that it didn’t work out as planned, and a significant part of mounted wings were not in position when the English advanced unexpectedly. Presumable they ended up with the rear battle.
But Le Fevre and Waurin were both present at the battle, and they say that the vanguard also had archers, in numbers that work out to one for every two men at arms. In addition, there were crossbowmen equal to a three for every eight archers. The absolute numbers they report are not reliable, but the ratios should be about right.
They add that the main battle had men at arms and archers equal to the vanguard.
We also know that the intent was to raise on archer for every two men at arms for the campaign, and this seems to have been achieved in the noble retinues we have pay records for.
We also know that gros valets were present, but not necessarily listed on the payroll. The original battle plan had them charging in support of the men at arms. Des Ursins suggests they were present in the main battle in addition to the men at arms and archers, and that there were crossbowmen there as well. Le Fevre and Waurin report that some who were struck down in the vanguard and main battle were rescued and led away by their servants. This is only plausible if those servants were no further away than the rear ranks of the same battle.
We don’t know how many valets were well enough equipped to play a useful role as a combatant. Later in the 15th c. it was expected there would be one similarly protected coustilier for every man at arms. A few years earlier, in his campaign against Liege John the Fearless had a bit under 4,000 combatants on his payroll. At the battle of Othee, he reportedly sent 1,000 gros valets in support of a flanking attack by 400-500 men at arms. His pay lists from around this time suggest men at arms were typically about 60% of the men on his army payroll, so a force of 10,000 men at arms might be able to also field at least 4,000 gros valets. And it’s possible that the number was higher, and there were additional gros valets at Othee that were not sent on the mounted attack. That does not seem unreasonable compared to the 1:1 ratio later in the 15th c.
So now we have:
Mounted wings: 1,200 men at arms
Vanguard: 4,800 men at arms, 2,400 archers, 900 crossbowmen, 1,000 gros valets
Main battle: 3,000 men at arms, 2,600 archers, 1,000 crossbowmen, 2,500 gros valets
Rear battle: 1,000 men at arms, 400 gros valets
Total: 20,400 combatants.
It was usual at this time for crossbowmen to be accompanied by pavisers, with about one paviser for every two crossbowmen. If not assumed to be included in the number of crossbowmen this would add another 900 men.
Since making the number of men at arms and archers the same in the vanguard and main battle would give an unusually high number of archers for French army of the period and contradict Berry Herald, I have taken La Fevre and Waurin's statement less literally, and that they meant that the total number of fighting men in the vanguard and main battles was about equal.
There would have been addition non-combatant servants. In the 14th c. two servants per man at arms, seems to have been typical. If we assume that only 3,9000 of them were combatant gros valets, that would have added another 14,000 pages and grooms. Of course, more of them might have been armed gros valets, which would keep the army size the same but increase the combatant numbers. Possibly some of the French servants were armed and carried on the payroll as archers, which would have reduced the total army size somewhat.
If we assume that English men at arms each had one yeoman serving as an archer and one page, that would only increase their total force by about 900 if the yeomen were already counted as archers.
This would then be a battle where an eyewitness could reasonable say that the French army was at least three times the size of the English. The chroniclers report that servants were exercising horses on the field before to the battle, and presumably many were assisting their masters prior to the combat, so they would have swelled the number included in a visual estimate of the army size.
There is another problem with Curry’s estimate of 12,000 combatants for the French army at Agincourt. This would imply, based on Berry Herald’s distribution of men at arms and the ratio of other troops reported by eyewitnesses, that the vanguard had 3,600 men at arms and 2,700 missile troops.
This would make the vanguard significantly smaller than the English army, and yet all three eyewitness reports say the reverse was true.
To push most of the missile troops in the vanguard to the rear, the men at arms would need to extend their front to cover about 750 yards. On this frontage, 4,000 men at arms would be less than four ranks deep, assuming 27 inches per man. 27 inches per man is within the range of Napoleonic era infantry doctrine, and fits with my experience with efforts to recreate medieval combat.
Four deep is a very shallow formation for a melee infantry unit by pre-gunpowder standards. The classical Greeks and Romans and the early medieval Byzantines all seem to have taken eight ranks as the norm. The English at Agincourt averaged somewhere between 4.5 ranks deep (conventional wisdom of about 6,000 men) and 6.75 (Curry estimate of about 9,000 English combatants.) Both Tito Livio and Pseudo Elmham say the English army was four deep. The English army was mostly archers, and might reasonably accept a shallower formation than a purely melee unit.