If you can get away with one, I think it is fine. I had to get 2 when I brought my kittens home. They are both female, they were both ok using the one box for urine, but one of the kittens would not do her poops in there. She always went to the patio door and did it right in front of my door. So I got another box. This worked out just fine.
I now have 4 cats, and 3 litter boxes. On all the websites I think they suggest 5 boxes for 4 cats. But since they are doing ok with 3, and I do clean them once a day, if not twice a day, they are ok.
I have both the covered and uncovered. They (or at least mine) do not have a preference.