What is the exact number of jātaka tales?
The Khuddaka Nikāya counts 547 Jātaka Tales.
Where can I read original Jātaka tales online?
At Sacred Texts there are 6 translated volumes that contain all of them.
It's Jataka #388 (Tuṇḍila Jātaka):
"Something strange to-day," etc. The Master told this tale while dwelling in Jetavana, concerning a brother who feared death. He was born in Sāvatthi of good family and was ordained in the Faith: but he feared death and when he heard even a little moving of a bough, or falling of a stick or voice of bird or
beast or ...