Is it necessary that in SN2 reaction if configuration of reactant is R then configuration of product is S?? Or is it like that if reactant is dextrorotatory then product should be levorotatory?

Yes it is necessary in Sn2 reaction that the change in configuration will take place because in Sn2 leaving group is leaving the same time when nuceolphile is attacking, therefore only back side attack is possible therefore change in configuration takes place. But one cannot say about levo and dextr because they are experimentally determined and they will form in pairs,i.e., we can say that both will form. No one is going to ask about dextro and levo,