Use the Independent Auto-Number Sequence Test Option

If this option isn’t enabled, there will be gaps in the auto-number field whenever Apex tests create test records with auto-number fields. For example, if Account has an auto-number field, and there are 50 account records in your organization, the field value of the last created account can be N-0050. After running an Apex test that creates five test accounts, this causes the auto-number sequence to be increased by five even though these test records aren’t committed to the database and are rolled back. Next time you create a non-test account record, its auto-number field value will be N-0056 instead of N-0051, hence, the gap in the sequence. If you enable this option before running an Apex test that creates test data, the auto-number sequence is preserved and the next non-test record will have a contiguous auto-number value of N-0051.

Gaps in the auto-number sequence can still occur in other situations, for example, when triggers that attempt to insert new records fail to execute and records are rolled back. In this case, gaps can’t be completely avoided because, in the same transaction, some records can be successfully inserted while others are rolled back.