How many variants of GSLV are there?

There are different variants of GSLV launch vehicles – GSLV Mk 1, GSLV Mk II, GSLV Mk III. The GSLV Mk I variant is no longer in service. You can read about the GSLV Mk III – A Comprehensive Overview of Heavy Lift Launch Vehicle in the given link.

Further readings:

1. Chandrayaan 2 Mission
2. Mars Orbiter Mission (MOM) – Objectives, Payloads