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.