So

SELECT

DES.login_name,

COUNT(DISTINCT DES.host_name) as login_hosts

FROM

sys.dm_exec_sessions AS DES

GROUP BY

DES.login_name

ORDER BY

login_hosts DESC

OR

DECLARE @login_name SYSNAME = {user name}

SELECT

DES.login_name,

COUNT(DISTINCT DES.host_name) as login_hosts

FROM

sys.dm_exec_sessions AS DES

WHERE

DES.login_name = @login_name

GROUP BY

DES.login_name

ORDER BY

login_hosts DESC

or what Luis posted while I was typing my reply.