Perhaps something like this?
SELECT l.userid,
COUNT(l.userid) AS nrlogins
FROM mdl_log l
WHERE l.module = 'user'
AND
l.action = 'login'
AND
l.time BETWEEN UNIX_TIMESTAMP('2013-01-01 00:00:00') AND UNIX_TIMESTAMP('2014-12-31 23:59:59')
GROUP BY l.userid