How to decide, whether a scene needs lightmap instead of realtime or not?

Is there any Guidelines or checklist, when a scene can manage with realtime lighting and when a scene requires light-map… though my primary requirement is for shadows, It will be helpful if the scope of answer cover other aspects of lighting also.

I’m not aware of any guide. The answer depends on what you’re trying to achieve and the targeted platform. Real-time requires more processing, so if you’re building for mobiles, it will need to be limited. Baked light maps will look and run great too. You can even mix the two. Either way, as long as performance is acceptable then there shouldn’t be a problem.

I don’t know what other aspects of lighting you want to know. Did you read any related articles by Unity?

Manual: Lighting Overview

Manual: Light Performance