Multiple SIMs are indeed common but I believe the drivers that caused people to have two SIMs will go down with time.
Unlike some other countries, India's mobile rates are very competitive (cheapest in the world, in fact) so there's little reason to shop around, so cost isn't a driver for multiple SIMs.
I was told of two primary reasons: One, India's telco regulator has "circles" which are basically zones in the country (some circles are essentially a state, but some of the smaller states may combine to form a circle). In-circle calling is at local rates. Out-of-circle calls had STD (long distance) rates applied. So you might have a 2nd SIM if you travel out of circle frequently.
Reliace Jio and Airtel ("the big two") have national licenses now, and I believe their users don't pay long-distance charges at all. So this driver to have two SIMs is disappearing.
The 2nd reason was -- different networks had coverage blackspots in different areas, and having two SIMs was an avoidance measure. Jio and Airtel, while not perfect, have fairly good 4G infrastructure and blackspots have significantly reduced.
Irrespective of the reason, the economic rationale for multiple SIMs was that a "pay as you go" SIM was very cheap to keep active. You paid a dollar or so and it was active for a long time -- 6 months or so (Jio never did this). With margins squeezed by Jio, other operators are feeling less charitable and want regular monthly payments or conversion to a plan to keep the SIM alive. The minimum payments are still ridiculously small especially in dollar terms, but they will pinch the less well off (given India's nominal average monthly income is about $140, there are a lot of people in this category). I suspect eventually most people won't bother if they're happy with their service.
There are other reasons (a lack of trust in the number portability process, India's lack of privacy legislation which means your number is up for grabs for spam calls and texts ignoring the regulator's Do Not Disturb guidelines) but until recently it was almost cost-free to have multiple SIMs. That'll change soon, I suspect.