The launcher alone has 16 Threads running. The game itself clocks in 30-40.
But most of them are propably just leftover diskreader threads or ones waiting for specific action to do anything at all. And even that will be far from a CPU bound operation.
You can start as many threads as you want. If they do not require a fair share of the CPU time (that the main thread no longer has to deal with), it will not speed up the whole process.
This does not answer the hypothetical question if it could be done and if it would be worth the effort. In theory, whenever you have a collection of objects, that are independent from each other, and you have to do something with each of them you can do such a task in parallel on multiple cores without much of a problem. However if what you are doing doesn't require much computing it is often not worth the effort unless you have a truly huge number of items in the collection.
With a lot more effort and care you can distribute even things that do depend on each other over multiple threads on different cores, but the efficiency you gain by doing that may be less than what you lose through the need to synchronise them, effectively slowing the program down by doing it. However it may be possible to design a game engine that actually is able to leverage multiple cores and translate that into better performance, though that is not always possible and certainly no trivial task.
FYI, devs already confirmed (3+ times in the one or other 'why doesnt Stellaris have multithreading yet') threads that Stellaris DOES use multi-threading. Just that there are a few critical core processes that cannot be multi-threaded, and those are the ones taking up the most time anyways.
As to how much of that is actual parallel computing, I can't tell, might be worth poking that question at them.
Of course Stellaris uses multi-threading, otherwise the game would have to be turn based. Very old games like Master of Orion 2 or Heroes of Might and Magic 3 may have been single threaded, I'm not sure, but any game that runs in real time needs multiple threads to listen for input from the player while the game does its thing or for being network capable.
The difference between multi-threading and parallel computing is, multi-threading on a single core means that things still happen one after another, each thread gets a certain number of cycles in which calculations happen, then the next thread gets some cycles and so on. Parallel computing means that those two threads get processed independently on different cores and don't compete for the same resource, this can make things faster but brings its own problems. Which is why it isn't necessarily an improvement. Certain things simply have to be done in a certain order, especially if a program isn't designed with parallel computing in mind. And even if you design a program with that in mind, it still doesn't guarantee that it would actually perform better than if it wasn't.
I trust that developers at Paradox know what they are doing. If parallel computing was the answer to performance problems, they would have solved it by now. As long as we can't look under the proverbial hood, all we can do is speculate.
