According to the universal system, the tooth that is denoted by the number 23 is

The tooth designated "1" is the maxillary right third molar and the count continues along the upper teeth to the left side. Then the count begins at the mandibular left third molar, designated number 17, and continues along the bottom teeth to the right side Ref - https://en.m.wikipedia.org/wiki/FDI_World_Dental_Federation_notation