Get count between start and end date

hnazzal Okay then you can try like below or you can share your PBIX file after removing sensitive data:-

_Attendees =
var start_date = min(calender[date])
var end_date = max(calender[date])
return SUMX (
    FILTER (
        'Table (6)',
        OR (
            (
                MONTH ( 'Table (6)'[Start Date] ) = month(start_date)
                    && YEAR ( 'Table (6)'[Start Date] ) = year(start_date)
            ),
            (
                MONTH ( 'Table (6)'[End Date] ) = month(end_date)
                    && YEAR ( 'Table (6)'[End Date] ) = year(end_date)
            )
        )
    ),
    [Attendees count]
)

Thank you, i will try this code

But what do you mean by 

[Attendees count]