In ServiceNow, the value in the Number field is auto-generated. The value in this field can be manually changed in System Definition >> Number Maintenance module.
Our application is able to globally restore the Number field for all out-of-box tables, and all custom tables. However, with custom scoped application tables, the Number field is not restored for any record that previously existed.
The recommended best practice for restoring the ServiceNow Number field in the application, is to create a separate external ID field, that you can map to, for the "Legacy Number".