It actually depends on the paper you use and the dividers you use. Circa paper is thicker than ordinary 18- or 20-pound paper. In my 3/4-inch letter-sized notebook, I have 5 plastic tabbed dividers and about 60-70 pages of paper, half of it heavier weight Circa stuff, half of it standard weight. It comes with 60 pages of heavy-weight paper, but I remember that adding dividers to what it came with made it more difficult to flip pages.
The good news is that while the Circa system overall can be a bit pricy, it is cheap and easy to change out the rings on a given notebook. Start with 1/2 inch rings or 3/4 inch rings, and then move up to bigger ones if needed. I have found that if the notebook is too full and then folded back on itself, some of the paper has a tendency to pop out of the rings.