Salesforce.com Unique Field Limitation

 

Salesforce have some of the limitation and one of it is number of unique fields per object. Salesforce only allow 7 fields as ExternalId per object and also consider Unique fields as ExternalId.

If you already have 7 fields which are marked as ExternalId or UniqueId or both then you will receive an error if you try to create a new field as Unique/External

 

and you try to make field unique it will show an error if that field in more than one record contains same value.

only Email, Text, Number fields can be marked Unique