Another thing you can do combined with what the others have already suggested, is "clever" camera work and prop placement, so you can create the illusion that there is a lot of people.

I ran into the same problem when making this image a long time ago by now.

I think there is 8-9 characters in total, which is not a lot when trying to illustrate an army, but as you can see, the main focus is the girl on the horse, so she is blocking the rest of the army, so what you see behind her, the spears, flags as well as those spears coming in from the side, is just spears placed so it looks like there is a person holding it. Gave them some huge shields that is held so they also block the view. I think it works quite well as an illusion.

So you could do something similar by making the shot slightly narrower, place some signs or whatever feels natural to your image and just show maybe a hand or feet like someone is just about to walk pass it. It can save you a lot of work, you have to remember it just need to look correct for that single shot :D