with a as (select * ,'STEP'||rank() over(partition by session_id order by ex_timestamp) c from process_log)
,b as (select * ,sum(case when process_id <> c then 1 end) over(partition by session_id order by c) d from a)
select 
  process_id PROCESS 
  ,count(case when d is null then session_id end) CNT
from b
group by 1 
order by 1 
;