I would like know what the max no of rows is in a clustered index (non-unique) on a
datetime column table in SQL Server 2008R2.
The number of rows is limited only by available storage. i.e. there isn't one.
I suspect it might actually be 9,223,372,036,854,775,807 rows (maximum size of a bigint). To put that number of rows into context, if you inserted a billion rows per day, it would take approximately 25,269,512 years to exhaust.
Update: please read comments below, as they talk specifically about the uniqueifier.