How to select max value in a column.

Use top1 with order by,

You may try like this,

select top 1 MAX(N_Days) days,COUNT(N_Days) count from tbl_setting
where N_flag=0
group by N_Days
order by count desc 

It will provide o/p like,

Days          Count
7             304

[Edit2:] If you need single column you may try subquery,

select A.days from (
select top 1 MAX(N_Days) days,COUNT(N_Days) count from tbl_setting
where N_flag=0
group by N_Days
order by count desc
) as A 

Output:

days
7

Hope this may helpful to you.

Try this,

select top 1 max(N_Days)days From tbl_setting where N_flag=0 group by N_Days order by  COUNT(N_Days) desc