Fun fact: India's population has grown by approximately 185m people since the last Purna Kumbh Mela in 2013.
It should also be mentioned that this year's event is the Maha Kumbh, the really big one which only occurs once every 144 years. This also explains why so many more participants are expected this year (250-400m) compared with last time (around 34m in 2013). Pretty much any faithful Hindu who has the health and the means will want to catch this one because it's literally a once-in-a-lifetime occasion.