One for each severity level. Here's how to test:

--create an alert, using severity 9

EXEC msdb.dbo.sp_add_alert @name=N'RWTest',

@message_id=0,

@severity=9,

@enabled=1,

@delay_between_responses=0,

@include_event_description_in=0,

@database_name=N'master',

@category_name=N'[Uncategorized]',

@job_id=N'00000000-0000-0000-0000-000000000000'

GO

-- now add a message that will have severity 10 by default

use master;

exec sp_addmessage 50001,10,N'Test Message 50001';

-- fire an event to the windows event log that will have severity 10

exec xp_logevent 50001,'rw test 1',INFORMATIONAL

--now go check the alert -> History tab, and you will see Number of occurrences has NOT incremented

--add an alert to use a severity 10 fire now:

EXEC msdb.dbo.sp_add_alert @name=N'RWTest2',

@message_id=0,

@severity=10,

@enabled=1,

@delay_between_responses=0,

@include_event_description_in=0,

@database_name=N'master',

@category_name=N'[Uncategorized]',

@job_id=N'00000000-0000-0000-0000-000000000000'

-- fire an event to the windows event log that will have severity 10

exec xp_logevent 50001,'rw test 1',INFORMATIONAL

--now go check the alert -> History tab, and you will see Number of occurrences has incremented

Hope this helps

-Richard