Hmm. They only show CPU utilization here, but it looks to me like they might be over provisioned and wasting a lot of money.
It could be memory bound. In which case CPU number could be drivn by something else like redundancy or memory limits.
Given the traffic the number of CPUs seems tiny regardless.
Probably running cron jobs so system has cpu spikes. But the metric shown here is sampled average
Amazing that even though Stack Overflow was the first of 100s of Stack Exchange sites, many which were killed off, that SO still has more daily traffic then all of the other sites combined; this based on the total traffic and SQL queries per day stats on the linked page.