Conversation so far:
who was the founder of buddhism and when was it founded
Buddhism was founded in the late 6th century B.C.E. by Siddhartha Gautama.
where was buddhism founded
Buddhism arose in the eastern part of Ancient India, in and around the ancient Kingdom of Magadha (now in Bihar, India).

Question: how many adherents
Self-contained rewrite: how many buddhists are there