What's the minimal set of cookware for Chinese, Japanese foods on an electric induction cook-top?
You can cook with only one pan. Or, if you want to go absolutely minimalistic, you can get the cheapest stainless steel pot you want and both cook and fry in it - not very convenient, but doable. In that sense, the minimum amount of vessels to buy is one.

Of course, your grandparents might find that they don't want to deal with the inconvenience of having only one pan, but nobody can predict which number of pans is optimal for them personally, this is something they have to decide.

Frankly, in your situation, there is no pan in the world that meets your grandparents' expectations. If they don't want to care for a seasoned cast iron or carbon steel, then this is out. If they hate food sticking, all other technologies beside PTFE and ceramic are also out. PTFE and ceramic are not durable, they fail after some time - and the higher the heat you use, the sooner they fail.

The way I have seen most people in your grandparents' situation solve the problem is to use PTFE only and replace it when it gets ruined - about once or twice a year.