Salesforce.com has put some limitations and one of those is the limit on No. of Unique fields per object. It allows only 3 External ID fields per object and considers Unique fields as type of external Id's only.
So, you can create maximum 3 unique fields per object but can get this limit extended by requesting the Salesforce.com via raising a Case.
ADM 201 Certification Question: - Only 3 External ID Fields can be created on an Object.
ADM 201 Certification Question: - Only Text, Email, Number fields can be made External Id fields.
Note: - You cannot make a field unique if it already contains data that is not unique.