Good question. Perhaps it's another Tolkien mystery? Personally, if I was to take a stab at this, I would say that maybe they didn't actually "fill" their area of attack, for instance the Deeping Coomb, to the brim. Remember how fast the Huorns are described to be. Pippin couldn't even make out their shapes because of their speed. It could have been the Huorns just moved so fast that it seemed as though they filled the place up to the brim. They were much faster than orcs, of course. 10,000 Huorns seems a lot to me as is. I can see no reason why they wouldn't be able to devour countless orcs in a matter of minutes, their speed acting as an illusion, looking like more Huorns than there actually were.
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"Loud and clear it sounds in the valleys of the hills...and then let all the foes of Gondor flee!" -Boromir, The Fellowship of the Ring
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