"I don't see any reason why volley fire would have to be 5000 archers; groups of a hundred or a few hundred is still a volley; groups of fifty even.
The assumption of absolutely synchronous loosing doesn't make sense either; a single command calling the archers to loose, when they then draw and release a single arrow in their own rythmn, would be close enough (especially for the only reason I can think of [see below]).
On that basis, hearing the command isn't important either, just follow the chap next to you."
If we get away from the idea of musket type volleys, then the 'volley' could easily be the few seconds it takes for any number of arcehrs to draw, as EA says, each archer draws in a way that may not be the same as everyone else. I can see the mechanical ease at which the first 'volley', ie the initial command to start the shooting off, but not in timed volleys, again, due to the mechanics of drawing and holding an arrow on string. It doesn't matter with reenactment battle arrows for the following key reason
the bows are lower draw weights, so can be held at draw for long, even though this is bad archery practice
Higher draw weights mean a heck of a lot of strain to maintain while waiting for the last man to draw, at which time the first to draw has wobbled his hand.
I have seen this even in battle reenactment archery, when I had to 'train' our lot to shoot in volleys, but with a change to the normal set of instructions. After some trial and error, we simply merged the 'draw' command with 'mark' and 'loose' as one combined command, the two seconds or so taken to say that, allowed the unit to draw in the same time and loose soon after drawing, ie they had in effect a two second window in which to draw and no strain on the bows.
We found this to serve us both in terms of the need for volleys in our hobby and to reduce the impact of the volley commands, ie they are not musket drill.
When we did not need volleys, it was easy enough to simply say 'loose x arrows', or better still for the captain to allow a certain amount of shots over time, then call 'fast' to stop them. The volleys are not any safer than free shooting, as long a you can control the shooting.
"But I can understand one reason that might get archers to loose in volleys: control of the ammunition."
You can do that simply by limiting the amount of time to shoot, see above.
"groups of a hundred or a few hundred is still a volley; groups of fifty even."
That is a large enough bunch of guys to take up a fair amount of space and then the captain would have to wait and be able to see when the last man had drawn. It is not easy with 20 men, let alone 100.
Can you imagine the overlap in commands in each discrete unit, each one waiting for the last man to draw?
I can see no advantage tactically for the commander to wait for a few guys to catch up, if anything I can see fewer arrows being shot because of the time lag of waiting, whereas a free shoot until told to stop would get more arrows in the air and have greater shock value and less chance to avoid arrows.
"Like English Archer, I'm not basing this only any documentation; simply logic applied to knowledge."
Our reenactment archery is a poor benchmark, given that most of us do not use a decent draw weight bow, so invariably base it on our ability to shoot 12 or whatever, heck, most of us could do that with a 30lb bow, could we do it with a proper warbow, even with training? On top of that, it is accepted good practice to not hold the draw when shooting, that is possibly the most significant factor, as it is a physical one, whose affects can be seen and measured, ie wobble (reduced accuracy) and maintaining bow strain at critical levels.