I was wondering what the minimum chain was to get a Shadow Pokémon
with the Shadow Radar. I got a gain of 90 for a Shadow Kakuna but
then it crashed after I selected a wrong place.
There's nothing like a minimum nor a max amount of chained pokemon
to assure a shadow. Also pokemon's rarity is not related to the
chain you find them.
Just to give an example, from my personal records, the fastest I've
catched was jigglypuff at chain #2 and the slowest one was pidgey
at chain #205