What is the optimum number of people in a weekly oncall schedule?
well, this is not your answer but I hope it helps ...
I lived both situations when I was single and no kids.
When the staff was short my boss tried to motivate us by talking about the extra money
and giving half day off. (I liked)
When the staff of complete I was getting on-call once a month and the money wasn't good enough to pay for the trouble. (I hated it)
I recommend to assess where your team is, personally and professionally, and put a few suggestion to be voted by them. Then give them the ability to self manage their schedule.