If I want to make sure I have fertile eggs for hatching, I keep a ratio of about 10 hens to 1 rooster. Since I have found my Bielefelder roos to be fairly mild mannered, I have successfully been able to keep 3 roos together, even a mix of breeds, with about 15-20 hens. That's about an average of 5-7 hens per roo.
I would say watch their behaviors. The fewer hens you have, the more they will be mounted. If they try to constantly run away from the roo, are acting afraid, or their feathers are getting worn in the saddle area, you may need more hens. Then there are times when the roos just have favorites and it doesn't matter how many hens you have. He'll stick with the same few hens and you'll have to put coverings on them.
You'd be safe to add more hens if you are able to, even other breeds would work in fine. I've had Rhode Island Reds, New Hampshires, Easter Eggers, Barred Rocks, White Rocks and Orpingtons in with my Bielefelders.
Have fun!