well, there are types of kumbh melas, the normal one is celebrated every 3 years in haridwar and prayag. and the other types are ardh kumbh mela which is celebrated once in 6 years and purna kumbh mela which is celebrated once in 12 years. the maha kumbh mela is celebrated once in 144 years and it was last celebrated in 2001 with a crowd of over 60 million which was largest gathering recorded in the history anywhere in the world.