So far, I've had the chairs turned away from the computers at the start of class. For the most part, whatever way they happened to sit when they came in is how they've been paired. I've not had a single complaint about how they are paired, and no problems with behavior.
We have three computers at the moment, and have had anywhere from three to twelve kids in the room at a time. We've done four kids to a computer for both the K-1st grade group and the 5-6th grade group. Not ideal, but it did work. Of course it's new, so to some extent they were just happy to be there regardless of the setup. A fourth computer will be coming soon.
I have found that I don't like having just one student at a computer. It happened a couple times (couldn't make myself leave one of the wonderful new computers just sitting there unused) and the students by themselves seemed almost lonely- the others were interacting and discussing what was going on, and the ones alone kept looking over to see what the others were doing.