How to create comment on table from query result
While I am not sure what you want to achieve (why it makes sense to store a count as a comment, especially if your table will get new rows (or lose some to DELETEs)), but here is how what your problem is and how you can solve it.

First, check what psql thinks you have in the variable:

\echo :list
selectcount(*)fromdepartment.master_data


Clearly not the result of your query.  To store that, you'll have to use \gset:

&gt; SELECT count(*) AS master_data_count FROM department.master_data;
 master_data_count
───────────────────
        1234567890
(1 row)

&gt; \gset
&gt; \echo :master_data_count
1234567890


If you would try to set the table description now, you'd get another error:

ERROR:  syntax error at or near "1234567890"
LINE 1: ...ABLE department.master_data IS selectcoun...


This is because the comment must be a (quote-delimited) text.  To get one out of your variable, you have two options.  One is adding escaped quotes to the variable using psql (using the same feature you misused setting :list: 

&gt; \set master_data_count '\'' :master_data_count '\''
&gt; \echo :master_data_count
'1234567890'


Or you can add the quote already in your query, so SELECT count(*) turns into

SELECT $$'$$ || count(*) || $$'$$


(I used dollar quoting above to avoid typing things like ''''.)

The result you can already use as a comment, as it contains the necessary quotes.