Count staff that move between roles

Hi jmilne320 
I hope this is what you're looking for

Active Staff = 
CALCULATE ( 
    COUNTROWS (
        FILTER (
            CALCULATETABLE (
                Pilots,
                CROSSFILTER ( Pilots[Staff Number], events[Staff Number], BOTH )
            ),
            MAXX ( RELATEDTABLE ( Events ), Events[Date] ) <= MAX ( 'Date'[Date] )
        )
    ),
    'Date'[date] <= MAX ('Date'[date] )
)

Hi jmilne320 
First you need to create a new column for end date in the vents table:

end date = 
MINX ( 
    FILTER ( 
        CALCULATETABLE ( Events, ALLEXCEPT ( Events, Events[Staff Number] ) ), 
        Events[start date] > EARLIER ( Events[start date] ) 
    ), 
    Events[start date] 
)

Then you need to deactivate or delete the relationship with the date table

The following would be the measure

Active Staff = 
VAR CurrentDate = MAX ( 'Date'[date] )
RETURN
    COUNTROWS (
        FILTER (
            Events,
            VAR StartDate = Events[start date] 
            VAR EndtDate = COALESCE ( Events[End date], DATE ( 3000, 1, 1 ) )
            RETURN
                StartDate <= CurrentDate
                    && EndtDate >= CurrentDate
        )
    )