Edit the general settings

Settings- Contacts

1. Click Contacts > General Settings - if using your Admin login or
   Click Settings > Contacts > General Settings - if using your User login.
2. Select the Auto assign numeric client code for new contacts check box to automatically assign a client code when manually creating a contact, .
   For new sites, or if you previously used alphanumeric codes, the automatic numbering starts at 1. 
3. Click Save.