How to use (COUNTIF) function in Power BI Desktop between 2 tables

Hi MhdMur ,
Based on your description, and based on my understanding, if there are duplicate values in Table II, I'm not quite sure if you need to count all Table I values that appear in Table II, or just how many unique Table I values appear in Table II i.e. de-duplicated. You can try the following two methods.
Here are my test data:

1. Calculate contains duplicate values

Count of Customer ID in Table 1 = 
COUNTROWS(
    FILTER(
        'Table1',
        NOT(ISBLANK('Table1'[Table1 customer ID]))
            &&
        CALCULATE(
            COUNTROWS('Table2'),
            FILTER(
                'Table2',
                'Table2'[Table2 customer ID] = 'Table2'[Table2 customer ID]
            )
        ) > 0
    )
)

2. Calculate without duplicates

Count = COUNTROWS(
    FILTER(
        'Table1',
        NOT(ISBLANK(
            LOOKUPVALUE(
                'Table2'[Table2 customer ID],
                'Table2'[Table2 customer ID],
                'Table1'[Table1 customer ID]
            )
        )
    )
)
)

Best regards

Albert He

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