There are different kinds of markets in India - small mom-and-pop stores (kirana stores), wholesale markets (mandis) - but individual consumers often buy there too, because prices are lower, and malls / supermarkets like in the West (from the last decade or two). India is also big and not homogeneous. So I can't give a general answer, only about what I know:
- first, fish, as has been said about the US in this thread, is not eaten much inland (though some fresh-water fish is), due to distances and heat and spoiling, and while India has a large coast, it has an even larger interior.
- second, I doubt there are even close to 100 varieties of fish available (in markets) at the coasts, maybe 20 to 40; those who know more, feel free to correct me, I don't live near the coast (though did long ago), and don't eat fish.
- third, I don't think that in any of the types of markets, I mentioned, all the same varieties would be available all year round (and kiranas don't sell fish or meat at all, except some may, but only tinned). It is a mix of some staples and some seasonal items.
>I doubt there are even close to 100 varieties of fish available (in markets) at the coasts, maybe 20 to 40
E.g. I've visited Goa (a coastal state) a few times, and at least at the beach restaurants / shacks, where they lay out the fish and other seafood varieties for inspection by customers, I don't remember seeing more than 20 or so items - kingfish, mackerel, red snapper, pomfret, crabs, a couple of types / sizes of prawns, squid, shark, that's almost it. One place that I went to a few times, had seafood special nights (the owner fished as a hobby), where there were a few more kinds, like barracuda, tuna, mussels, etc. These were in general touristy areas though. Local markets and restaurants may have more varieties.
- first, fish, as has been said about the US in this thread, is not eaten much inland (though some fresh-water fish is), due to distances and heat and spoiling, and while India has a large coast, it has an even larger interior.
- second, I doubt there are even close to 100 varieties of fish available (in markets) at the coasts, maybe 20 to 40; those who know more, feel free to correct me, I don't live near the coast (though did long ago), and don't eat fish.
- third, I don't think that in any of the types of markets, I mentioned, all the same varieties would be available all year round (and kiranas don't sell fish or meat at all, except some may, but only tinned). It is a mix of some staples and some seasonal items.