Do you mean variants of Tejas or of different fighters overall?
I wasn't aware it was the former. If the latter I guess that's a result of trying to boost the Indian airforce as quickly as possible from multiple countries, without sacrificing the domestic industry.
In any event, no doubt there will be more commonality into the 2030s as large numbers of single types of aircraft are procured.
Tejas, ORCA, TEBDF, Tejas Mark 1A, Tejas 2, AMCA.
Coupled with Jaguars, Bisons (probably retired in next few years), MKIs, MiG-29 (at least two variants), Mirage-2000, and Rafales. Possibly F-15EX, Super Hornet, and/or F-21IN.
If I’m not mistaken different Tejas variants already use different engines (F-404 vs F-414).