with
r as (
  select
      SESSION_ID
    , PROCESS_ID
    , EX_TIMESTAMP
    , row_number() over(partition by SESSION_ID order by EX_TIMESTAMP asc) as n
  from
    PROCESS_LOG
), 
v as (
  select
      SESSION_ID
    , PROCESS_ID
  from
    r
  where
      (PROCESS_ID = 'STEP1' and n = 1)
  or  (PROCESS_ID = 'STEP2' and n = 2)
  or  (PROCESS_ID = 'STEP3' and n = 3)
  or  (PROCESS_ID = 'STEP4' and n = 4)
  or  (PROCESS_ID = 'STEP5' and n = 5)
) 
select
    PROCESS_ID
  , count(SESSION_ID) as CNT
from
  v
group by
  PROCESS_ID
order by
  PROCESS_ID asc
;