DECLARE @srcRc INT;
DECLARE @dstRc INT;

SET @srcRc = ( SELECT COUNT(*) FROM A )

INSERT  INTO t
        (Col1
        ,Col2
        ,Col3
        )
        SELECT  A.Col1
               ,A.Col2
               ,B.Col3
        FROM    A
                JOIN B
                    ON A.Id = B.Id;

SET @dstRc = @@ROWCOUNT

DECLARE @srcRc INT;
DECLARE @dstRc INT;
SET @srcRc = ( SELECT   COUNT(1) FROM A );
SET @dstRc = ( SELECT   COUNT(1) FROM A JOIN B ON A.Id = B.Id );