Maximum number of External IDs per object or entity which you can designate is 25 fields per object. External ID fields must be Custom text, number or email fields. External ID fields contain record IDs from systems outside Salesforce.
You can designate up to 25 External ID fields per object.
External ID fields must be Custom text, number or email fields.
External ID fields contain record IDs from systems outside Salesforce.
You can use the upsert call to match against External ID fields during import or integration.
External ID fields are indexed, so selective filters on them should run quickly.