Adding Customer Order Number Column

I'm working with a table that has unique Customer ID's and Order IDs, but has one row per item in a given order.  I'm trying to add a calculated column to the table that will tell me whether the row belongs to their first order, second order, etc...  

Here's a simplified example table with the desired Order # column that I'm trying to add.

I can't figure out how to add a count based on both a unique customer ID and for that customers unique OrderID.

I've been trying variations on this code, but whenever I add in customerID it either crashed Power BI or returns something else that's not what I need.

Order# = 
VAR CurrentDate = Sales[order_date]
RETURN COUNTROWS (
    FILTER(
           CALCULATETABLE (
                Sales, 
                ALLEXCEPT(Sales, Sales[customerId])
             ),
        Sales[order_date] < CurrentDate
          || ( Sales[order_date] = CurrentDate )
    )
)

Any help would be much appreciated!  Thanks!