can unique column accept multiple null value?

There is a difference between Unique key and unique index.
A unique key is a unique index but Unique key can have only one null value and unique index can have multiple null values, try this:

Unique key:
CREATE TABLE tbl_TestUniquekey (ID INT)
GO
ALTER TABLE tbl_TestUniquekey
ADD CONSTRAINT uk_id UNIQUE NONCLUSTERED (ID )
Go
INSERT INTO tbl_TestUniquekey
VALUES (1),(NULL),(NULL),(2),(NULL)
/*output:
Msg 2627, Level 14, State 1, Line 29
Violation of UNIQUE KEY constraint 'uk_id'. Cannot insert duplicate key in object 'dbo.tbl_TestUniquekey'. The duplicate key value is (<null>).
The statement has been terminated.
*/



Unique Index:

CREATE TABLE tbl_TestUnique (ID INT)
GO

CREATE UNIQUE NONCLUSTERED INDEX idx_tbl_TestUnique_ID
ON tbl_TestUnique(ID)
WHERE ID IS NOT NULL
GO
INSERT INTO tbl_TestUnique
VALUES (1),(NULL),(NULL),(2),(NULL)

/*Output:
(5 rows affected)
*/