count this week

Assuming you don't have a calendar table and at this point you only have dates and not time to worry about you could try:

CountOrdersThisWeek :=
CALCULATE (
    COUNTROWS ( 'OrdersTable' ),
    FILTER (
        'OrdersTable'[orderdate],
        [orderdate] <= NOW ()
            && [orderdate]
                >= TODAY () - 7
    )
)

I would try and avoid anything in a calculated column that refers to NOW() or TODAY() simply because it is only evaluated when the model is loaded (data is refreshed) and subsequently could be incorrect if your data wasn't updated daily and or failed for some reason.

Hope this helps some.

Thomas