with cte as
 (
  select
      case
        when cast(Time_In as time) >'12:00:00' and cast(Time_In as time) <='15:00:00'  and Visit_Date = cast(GETDATE() as date)then 'Session 2'
        when cast(Time_In as time) >'3:00:00' and cast(Time_In as time)<= '6:00:00' and Visit_Date = cast(GETDATE() as date) then 'Session 3'
        else 'Session 1'
      end session
      /*course*/
    from Lab_Visits2
  ), ctecnt as (
  select session,/* course,*/ count(*) cnt
  from cte
  group by session /*course*/
)
select session, /*course,*/ (cnt)
from (
  select session, /*course,*/ cnt, row_number() over (order by cnt desc) rn
  from ctecnt
  ) t
where rn = 1