Cloud Kicks wants to display the number of Opportunity records associated with each Account.

Use a Roll-Up Summary field on Account to count the related Opportunity records. Salesforce roll-up summary fields can calculate values from related records, and the COUNT summary type totals the number of related records. Trailhead also gives the Account and Opportunity example for roll-up summaries.

A formula field cannot count child records by itself. A lookup field only creates a relationship. AppExchange is unnecessary for this standard use case.

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