Christian1971, I think you asked good questions and got a good answer.
I plant dahlias, standard size plants, at about 30 per 100 square feet. That's a little crowded but a compromise when choosing which tubers will be used in the spring.
Your 8' by 8' bed seems a little crowded by those standards. 8' by 16' is probably more than they need. Plan on an expansion next year.