How many External ID’s can have for each salesforce standard or custom object?

-Saelsforce custom object, we can create up to seven (7) custom fields on each object as being an external ID field.
-The field type should be a text, number, or email datatype.

What is the use of Salesforce External ID’s?

-An Salesforce Custom objects, External ID contains record IDs from a system outside (SAP, ORACLE or any other CRM ID, etc) of Salesforce.
-We can match against this external id field during insert/update or integration, or when using the upsert call (Insert/Update).
– External ID fields are indexed, so selective filters should run quickly and efficiently.