If the area code number order does not matter you can try this:

1. Calculate the number of similar "REGION" features with count(expression, group_by, filter)

2. Order the shapefile according to "REGION". Its possible to do by MMQGIS plugin "sort-tool"

3. Make unique "id" for all areas in field calculator with @rownumber

4. Count numbers for areas in field calculator

"id" % "counted_areas" +1

If the area code number order does not matter you can try this:

1. Calculate the number of similar "REGION" features with count(expression, group_by, filter)

2. Order the shapefile according to "REGION". Its possible to do by MMQGIS plugin "sort-tool"

3. Make unique "id" for all areas in field calculator with @row_number

4. Count numbers for areas in field calculator

"id" % "counted_areas" +1