SQL & PL/SQL :: How Many Times Do The Inner Execute

I found this query in one of my stored procedures that updates a key for a value in a data table by reading the information from a master table.

The data table is: ITEM_INVENTORY
The master table is: MASTER_SOURCE_SYSTEM

UPDATE ITEM_INVENTORY I
SET I.SOURCE_SYSTEM_ID =
(SELECT NVL(M.SRC_SYS_ID,-100)
FROM MASTER_SOURCE_SYSTEM M
WHERE M.SRC_SYS_DESC(+) = I.SOURCE_SYSTEM_CODE )
WHERE ORG_CODE = 'TNXC'
AND EXISTS (SELECT 1 FROM MASTER_SOURCE_SYSTEM M
WHERE M.SRC_SYS_DESC(+) = I.SOURCE_SYSTEM_CODE )

The situation here is that:

1. There are about 15000 rows that match ORG_CODE = 'TNXC'.
2. The SOURCE_SYSTEM_CODE is same for all the 15000 rows and there is a matching entry for it in the MASTER_SOURCE_SYSTEM table.

My question is: Do both the inner select statements execute 15000 times?

The statement executes within a second and updates 15000 rows. How is this made possible?