400 billion stars (approximately). 100 billion star systems (again, approximately).
Like reality, most systems have more than one star in them. A large number are binary... but there are trinary and quaternary (more?) also.
While there are systems with more than four stars (well, I think there are...), the average stars per star system should quite smaller - therefore the 100 billion star system and 400 billion stars doesn't seem coherent.
While afaik there is a lack of clarification, for the time being and taking FD/DB statements on the subject, I think there are almost 400 billion star systems in the game. Yes a ridiculous number