I'll let you in on a secret, none of us know!
Procedural generation is like that, we can ballpark it, we know how much mass there is, how the algorithms work, etc. But the only way to get an exact count is to generate the entire galaxy and count them, which can of course be automated, but isn't a quick task.
There's probably tools the stellar forge peeps have that could do this, but every time the rules change a little, that number can change a lot.
There's enough that the exact number isn't particularly meaningful tbh.
Since I bought the game on the belief that it contained 400 billion stars, if I find less than that, can I have a refund on the basis that it was missold to me ?