When you have loads of cats it's hard to stick to this, but the rule of thumb is that you should have one more litter box than cats.
We have six cats, three litter boxes and one automatic litter box upstairs (one in each room except one room has two) and three litter boxes and one automatic litter box downstairs.
We scoop once a day, and we dump and bleach the boxes once a month.
When we lived in the RV we had just five of the kitties and only three boxes. We scooped twice a day and dumped and bleached the boxes every week.
We used to use the Arm & Hammer - but it's very dusty. We then tried to switch to the eco- and health-friendly stuff, like Swheat Scoop or The World's Best Cat Litter - but we live in rural areas, and both attracted ants.
So we now use TidyCats Crystal Blend - the antibacterial one (small pour plastic box with green label). It clumps really well for the most part, and while there's dust, it's not nearly as bad as the Arm & Hammer stuff was.