Instances per computer
 50 instances on a stand-alone server for all SQL Server 2005 editions except for Workgroup Edition. Workgroup Edition supports a maximum of 16 instances.
SQL Server 2005 supports 25 instances on a failover cluster.
 50 instances on a stand-alone server.
25 instances on a failover cluster.
 

http://msdn2.microsoft.com/en-us/library/ms143432.aspx