Contact serial number generation is defined by a Sequence. The sequence handles the structure, current number, and future numbers when creating a new contact.
The sequence can't be changed in CRM via a setting however the support team can update the existing sequence if needed.
Please reach out to us via the Digital Assistant and provide us with as many details as possible so we can investigate and assist.
📌 Note: Typically, a sequence can only be moved forwards, not backwards. This is to prevent duplication errors against existing contacts as the sequence moves forwards.