What is the maximum number of external IDs an object may have?
A. 1
B. 2
C. 3
D. 4
E. 5
Answer: C. 3
For each object that can have custom fields, you can set up to THREE custom text, number, or email fields as external IDs. An external ID field contains record identifiers from a system outside of Salesforce.
You can use an external ID field to update or upsert records using the API. When using the API or import wizards for custom objects and solutions, you can use this field to prevent duplicates by also marking the field as Unique.
Note: Custom fields marked as Unique count against an object's limit of three External ID fields. Custom indexing that occurs automatically in the background by Salesforce does not count against External ID limits.