The Number field is not available for Field Mapping when performing a CSV Import of Support Cases

Consider the following:
 
- A CSV Import of Support Cases is being performed in Setup > Import/Export > Import CSV Records > Import Type = Support, Record Type = Support Cases  

- Auto-Generated Numbers is enabled for Support Cases in Setup > Company > Auto-Generated Numbers > CRM subtab > Support Case = True

To be able to map the Number Field, disable Auto-Generated Numbers for Support Cases during the time of import:

1. Navigate to Setup > Company > Auto-Generated Numbers
2. Under the CRM subtab > Support Case = False
3. Save

After performing the CSV Import, enable back Auto-Generated Numbers for Support Cases.