Replace values based on measure

hi,  Anonymous 

I've had a case like "top n and others" before

https://community.powerbi.com/t5/Desktop/Top-N-and-Others-monthly/m-p/490708#M228672

For your case, "owners with only one car" must be defined in a column first.

Because measure is an aggregate value and dynamic based on the row context, 

Then use this formula to create a measure

Measure = 
VAR _table =
    SUMMARIZE (
        ALLSELECTED ( Table1 ),
        Table1[customer],
        "_countcar", CALCULATE ( COUNTA ( Table1[car] ) )
    )
RETURN
    IF (
        SELECTEDVALUE ( Specialties[customer] ) = "owners with only one car",
        CALCULATE ( COUNTAX ( FILTER ( _table, [_countcar] = 1 ), [customer] ) ) + 0,
        IF ( [countcar] <> 1, [countcar] )
    )

Result:

By the way, you could also add or adjust the conditional of the measure.

here is pbix file, please try it.

Best Regards,
Lin