I have two boxes for 8 cats. I just added a third box over the holidays as some of our permanent cats are too large (pronounced "fat") to fit in a "normal" sized box. That's NOT the standard though. General rule is 1 box per cat, plus one spare, so technically I *should* have 9 boxes. I shudder to think how much time I'd spend cleaning that many. My guys get along with 2 (now 3) though, and no messes, so it works for us.
We clean ours once a day (fosters always get cleaned twice a day if we have them in a separate room with a separate box). I love my clay/clumping litter. I feel it controls the smell much better than the pine stuff, as my cats who've used the pine stuff weren't interested in burying it at all. Also, people who come to my house frequently comment on the good smells from the kitchen, and I don't use air fresheners or anything, so I think my clumping stuff does a good job controlling odors.
Generally cleaning once a day is a good rule. To some extent, you have to find what works for you and your cats. If they're making messes where they shouldn't, try scooping more often (twice a day, three times)or adding more boxes to see if that helps. If you're not getting much when you scoop, you might be able to reduce the frequency of scooping, or the number of boxes. It's a trial and error kind of thing, to an extent.