I’m calling this article, “How many employees by period”. Staff come and go for different reasons. In some companies, the number of staff can change over time. The principles used in this article can also be used in other instances. There can be staff moving in and out of departments, on and off of projects, etc. The technique can also be used to work out how many staff were on leave, how many off sick, how many tickets were open in a support queue, or any other concept that has a start and end date in a transactional table.
Read on for Matt’s answer but be sure to check out the comments as there are some other good solutions in there.