I use Vario sheets of various sizes, mostly in large 4" D-Ring binders. A 3S sheet will hold 3 #6 covers per side, so six total. All my covers are otherwise already in clear cover sleeves, so in situations where it is not as important to me for the full front of a cover to be shown, I will often use Vario 5S sheets, which allow 4 covers overlapped per side. I keep a few 1S and 2S for various odd sized covers. I find this to be a relatively inexpensive way of managing a rather large collection of covers (20 binders at last count, though some are 3", stuffed full and in need of being replaced with 4" binders).
If pictures are worth 1,000 words...
Using Vario 3S pages:

Using Vario 5S pages:
