To be honest, I'd have to have another look at the film. I've seen it many times, but not recently. As I recall, the rampart down which Theoden, Aragorn, & company ride is not very wide. A war horse, fully accoutered and mounted by an equally fully armored rider, takes up a bit of space, so I'm not sure how many more soldiers could reasonably have followed at once.
"And it is said by the Eldar that in water there lives yet the echo of the Music of the Ainur more than in any substance else that is in this Earth; and many of the Children of Ilúvatar hearken still unsated to the voices of the Sea, and yet know not for what they listen."
There is something profound about standing AT sea level.
