The maximum number of related records returned for each base record varies depending on the calling function:

Appian’s Data Sync feature (for record types with sync enabled) has several built-in limits:

Synced Row Limits

Depending on your version of Appian:

Up to ~50 million rows per record type can be synced from your source system (e.g., database or web service) in recent releases in 25.4. Appian Documentation+1
When applying a relationship filter on sync, the record type has a lower limit (~10 million rows). Appian Documentation
Older documentation cited ~4 million row limits; newer versions have increased this significantly. Appian Documentation+1

API & Sync Smart Service

If your record type uses Salesforce as a source, 1,000 rows per 1 API call is required. Appian Documentation
The Sync Records smart service can sync up to 1,000 specific rows at a time. Appian Documentation