About 800 years ago, Pillaiperumal Aiyyangar composed 108 Tirupati Andadhi. Elders used to say the tradition of 108 divya desa count started.
There are no historical refence for such a count or list before this. After his period, several books on 108 divya desams have been released. Even in these, some count varies. For the sake of counting, we consider the sthalams that Azhvars have sung upon. There are no definitive rule for this count.
For example, it used be said that in the olden days, there were disputes whether to include Tirunarayanapuram and Kanchipuram Varadaraja Perumal temple as part of 108 divya desam.