Not a bad metric. After about 4 people or so, it might as well be "open office". That is the chance of each one of those people being distracted by someone from outside the office, or a phone, or two of them talking about something is pretty high.
(Worked in a cubicle farm, 4 person office, 3 person office, 2 person, then my own office, now at home in my own office).
Top choice is work at home. Then own office. Then 2 person office. Once it gets to 3 person office, I'd pick a cubicle with high sound insulated walls.
Open office would be a nightmare for me. I'd get set just from stress of not being able concentrate and get my work done.