personid, msg
--------------
 1,      'msg1'
 2,      'msg2'
 2,      'msg3'
 3,      'msg4'
 1,      'msg2'

select distinct personid, count(*)
FROM mytable;

select distinct personid, count(msg)
FROM mytable;

id, count
--------
1,   2
2,   2
3,   1

SELECT personid, COUNT(msg)
FROM mytable
GROUP BY personid
ORDER BY personid;