Conceptually, I’m looking for a combination of wp post list –format=count and wp site list –format=count but I don’t think that’s feasible.
Nope, such a command does not exist, but adding it would be wasteful and counterproductive.
Just because there isn’t a dedicated single tool that builds a house all at once doesn’t mean tools don’t exist, or that you can’t build houses.
It’s the same here. Multiple commands combined together can give you what you want.
For example that is no command that will print out the name of all active blogs, and yet something similar to these commands will do that:
blogs=$(wp site list --fields="url" --format="csv")
for url in $blogs
do
wp option get blogname --url="${url}"
done
Note that the above is standard bash scripting, it’s not unique to WordPress.
Likewise storing the post count in a variable with COUNT=$(wp ..... ), and adding 2 variables together with basic addition using num=$((num1 + num2)) should give you the missing pieces to complete what you want and get a count that includes all posts across a multisite.