The request number shows "UNKNOWN" on form header of sc_request. Another symptom can be in the requested item(RITM), "Request" field displays "UNKNOWN" instead of the request number.
Another scenario can be if the incident number is not visible on the form header.
The display field is turned off in the sys_dictionary entry of Number field in the Task table. Since display is disabled on the parent table the number is not visible on the extended tables. Thus number is not visible on request/incident form.
To resolve the issue enable the display checkbox in the sys_dictionary entry of Number field in Task table.