Storing Results Into Variables

I need to manage the number of rows affected by a query.
If I run this script on my database:
SELECT COUNT(*)
FROM TABLE_NAME
WHERE TABLE_NAME.FLAG = 'I'
AND NOT EXISTS
(SELECT*
FROMERR_TABLE_NAME
WHERE ERR_TABLE_NAME.FIELD_1 = TABLE_NAME.FIELD_1
AND ERR_TABLE_NAME.FIELD_2 = TABLE_NAME.FIELD_2)

the query analyzer gives me back the number of lines affected:

-----------
4,00

(1 row(s) affected)

I need to manage this amount (4,00), so I applied this modification:

DECLARE @NUM_ROWS INT

SET @NUM_ROWS = (SELECT COUNT(*)
FROM TABLE_NAME
WHERE TABLE_NAME.FLAG = 'I'
AND NOT EXISTS
(SELECT*
FROMERR_TABLE_NAME
WHERE ERR_TABLE_NAME.FIELD_1 = TABLE_NAME.FIELD_1
AND ERR_TABLE_NAME.FIELD_2 = TABLE_NAME.FIELD_2)
)

but it doesn't work!! the SRV returns me this:
Server: Msg 107, Level 16, State 3, Line 3
The column prefix 'TABLE_NAME' does not match with a table name or alias name used in the query.

Do you know another way to store the number of rows affected?