Create columns - Number on roll using start and end dates

I think I understand what you want to do.  A few steps to this idea:

1. Relate the Staff table to a Calendar or Date table on the StartDate. 

2. Create a measure counting the the entries in your staff column that are active in that month:

ActiveEmployeeCount = 

VAR MinDate = Min(DateTable[Date])
VAR StartDate = SelectedValue(StaffTable[StartDate])
CALCULATE(CountRows(StaffTable), ISBLANK(EndDate), StartDate >= MinDate)

3.Create a matrix visual using your StaffID as rows and date table month as columns.  Place the new measure in the values.