DBCC TRACEON
(3004, 3605, -1)
http://www.red-gate.com/supportcenter/Content.aspx?p=SQL%20Backup&c=knowledgebase\SQL_Backup\KB200806000267.htm
Running with trace flags 3605 & 3213 will result in output
to the ERRORLOG containing the number of buffers used for backup
operations.