Maximum number of 'External Ids' per object or entity

When unable to mark a custom field as External

1. Make sure there are fewer than 25 External ID fields on the object.
2. If there are more than 25, you may need to log a case with Salesforce Support.

Notes: 

• When a field has been designated as an External ID, it is also indexed.
• The maximum number of custom indexes per entity is 25, by default.

To request a review and increase of index operations

1. Ask your administrator to log a case with Salesforce Support.
2. Provide your organization ID.
3. Explain the business need for increasing the limit on indexes.
4. Salesforce Support will review your request and current indexes, then action as needed.