That is fantastic, thank you Jeff. I'm a bit annoyed I didn't pick up on that in the documentation.
I'd thought about using is.culled() function before for counting the number of objects in a scene but I do have clutter (and therefore, a lot of obscured objects) which, unfortunately, means I must use the more manual method outlined in my first post. Fortunately, I do not need a super-fast, efficient way of counting objects in a scene.