The synced record type Customer has a one-to-many relationship with the Case record type.

Within the Case record type, edit the relationship to the Customer record and select the "Open” status as the grouping filter.

Within the Customer record type, create a custom field that evaluates in real-time to aggregate related record fields and count the open cases.

Create a query expression for the Customer record type. Utilize the a!measure() function within the aggregation fields to count the related cases in the “Open” status.

Create a sync-time custom record field within the Customer record. Utilize the a!relatedRecordData() function to filter open cases and count the number of entries.