create table tb 
(
col1 varchar(10),
col2 varchar(10),
col3 varchar(10),
col4 varchar(10),
col5 varchar(10)
)

insert into tb
select 'col',null,null,'col4','col5'

GO
declare @String varchar(max)
SELECT @String=CONVERT(varchar(20),isnull(col1,'@'))+CONVERT(varchar(20),isnull(col2,'@'))+CONVERT(varchar(20),isnull(col3,'@'))
      +CONVERT(varchar(20),isnull(col4,'@'))+CONVERT(varchar(20),isnull(col5,'@'))
from tb

select datalength(@String)-datalength(REPLACE(@String,'@','')) as 'NULL值个数'

GO
DROP TABLE TB