Hello Saumya Banerjee, The number of seats in IIT's changes every year, it is not constant every year, every year some seat are increased and some are decreased depending upon the scenario.
As far as for Geology it is present in very less IIT's so its seats are very less.

For Example

IIT Kharagpur has 43 seats during 2019-20 session, this year it has chances to increase some seats.

IIT Dhanbad has 18 seats.