If I have figured correctly, what you are after:

; WITH cte AS
  ( SELECT pid, event, timeStamp,
           maxts = MAX(timeStamp) OVER (PARTITION BY pid, uid)
    FROM tableX
  )
SELECT pid, cnt = COUNT(*)
FROM cte
WHERE maxts = timeStamp
  AND event = 200 
GROUP BY pid ;

If I have figured correctly, what you are after:

; WITH cte AS
  ( SELECT pid, uid, event, timeStamp,
           maxts = MAX(timeStamp) OVER (PARTITION BY pid, uid)
    FROM tableX
  )
SELECT pid, cnt = COUNT(*)
FROM cte
WHERE maxts = timeStamp
  AND event = 200 
GROUP BY pid ;

If there is a chance that you have 2 rows with identical pid, uid and timestamp, you'll need to replace COUNT(*) with COUNT(DISTINCT uid).