Why for monolayer graphene I2D/IG value is 4 and for bilayer graphene it is 2? What are the physical significances behind this? All scientific answers and explanations related this are highly appreciable.
The classic paper about Raman on graphene is this here, although it doesn't make an explicit statement on your question yet:
Article Raman Spectroscopy in Graphene
The ratios that you cited are not applicable under all circumstances, they are empirical values which are found under certain criteria. The first criterium is the flake size: if you have only nanoplatelets, the G mode intensity may shrink and you would get different ratios. Also, your material must have little to no defects, because otherwise the D mode goes up and the G mode goes down.
Using exclusively this ratio for the analyses is therefore risky, but you can double-check by also analyzing the position of the G mode and the shape of the 2D mode: https://assets.thermofisher.com/TFS-Assets/MSD/Application-Notes/raman-spectroscopy-graphene-determination-layer-thickness-an52252.pdf
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