In Jain tradition, the story of elephants observing a fast is associated with the sacred site of Hastigiri (also known as Hastinapur or Hastigiri Teerth). According to Jain scriptures, particularly in the context of the legendary story of Shantinath Bhagwan (the 16th Tirthankara), it is said that one thousand (1000) elephants observed a fast at Hastigiri.

This event is celebrated in Jain lore as a demonstration of the power of Jain teachings, where even animals, inspired by the aura and compassion of the Tirthankara, engaged in spiritual practices such as fasting (upvaas).

So, the answer is: One thousand (1000) elephants observed fast on Hastgiri, according to Jain tradition.