import arcpy
from collections import defaultdict

fc = r'C:data.gdbfeatureclass'
fields = ['number','category','combined']

d = defaultdict(list)
with arcpy.da.SearchCursor(fc, fields[:2]) as cursor:
    for num, cat in cursor:
        d[num].append(cat)
#d will now look like {1112:['A','A','B','B'],555:['B','B'],...}

with arcpy.da.UpdateCursor(fc,fields) as cursor:
    for row in cursor:
        row[2] = ', '.join(sorted(list(set(d[row[0]]))))
        cursor.updateRow(row)