Hi Rahul, No requirement if you are staying in transit zone. As you won't be leaving the airport, you won't need to go through any formalities. ... If you're transiting air side at Bangkok Suvarnabhumi Airport (BKK) with both the inbound and outbound flights on the same ticket you won't need any form of visa.
Just check if your connecting flight is from BKK or DMK, if Mumbai to bkk and dmk to Phuket then you will need it.. if its only bkk airport then mostly you don't need as long as you are going thru transit area..just check with airline if you have to collect luggage and go thru transit area only