How to count null values from multi-columns

The usual approach is to use CASE:

SELECT CASE WHEN field_1 IS NULL THEN 1 ELSE 0 END
       + CASE WHEN field_2 IS NULL THEN 1 ELSE 0 END
       + ... AS total_nulls,
       COUNT(*) as num_rows
FROM table 
GROUP BY total_nulls;

With the grouping and field names from your question:

SELECT MPAN, Date,
       SUM(CASE WHEN Reading1 IS NULL THEN 1 ELSE 0 END
       + CASE WHEN Reading2 IS NULL THEN 1 ELSE 0 END
       + ... ) AS total_nulls
FROM table 
GROUP BY MPAN, Date;

SELECT
  MPAN,
  Date,
  NullCount = COUNT(*) * 48
    - COUNT(Reading1)
    - COUNT(Reading2)
    - COUNT(Reading3)
    …
    - COUNT(Reading48)
FROM atable
GROUP BY MPAN, Date

Use a case statement to compute the sum:

select MPAN, date, sum(null_cnt) from (
  select MPAN, date,
  (case when val1 is null then 1 else 0 end) +
  (case when val2 is null then 1 else 0 end) + ...
  (case when val48 is null then 1 else 0 end) null_cnt
  from ...
) group by MPAN, date

I'm afraid you'd have to do something ugly like

select MPAN, Date, (
    CASE WHEN Reading1 is null then 1 else 0 end +
    CASE WHEN Reading2 is null then 1 else 0 end +
    CASE WHEN Reading3 is null then 1 else 0 end +
    CASE WHEN Reading4 is null then 1 else 0 end +
    ... +
    CASE WHEN Reading48 is null then 1 else 0 end
) as CountOfNulls

from YourTable

If you can change the schema, the first thing I would do is break this up into three tables:

'Readings' with an ID and an Name, values like

ID  Name
1   Reading1
2   Reading2
...
48  Reading48

This makes it easy to add more as you undoubtedly will.

Next, a 'MPAN' table with the 'MPAN's with ID, MPAN and date:

ID MPAN Date
1  134  21/05/11
2  134  22/05/11

And last, a 'Score' table that relates the two and includes the scores with ReadingID, MPANID, and Score:

MPANID ReadingID Score
1      3         0.345
1      48        0.789
2      48        0.467

Then you can let relational databases do what they do best and leverage the relationships. For example, this query would get all the MPAN's that had a score for Reading2:

SELECT MPAN.*, Scores.Score
  FROM MPAN
  JOIN Scores ON Scores.MPANID = MPAN.ID
             AND Scores.ReadingID = 2

USE THIS

select 
MPAN, Date,
coalesce(isnull(reading1,0)+isnull(reading2,0)+isnull(reading3,0)+isnull(reading4,0),0) 
from 
tablename