If i'll appear in improvement exam in 2019 and also in jee mains then in jee main form which roll no. Of board exam Should be used.

During filling of jee main form they will ask ( passed or appearing ) and also you will have to provide you previous years marks but then you have to select the option that you will be appearing for improvement exam but no need to worry about that as you may provide either year roll no. or roll no of marks sheet which is having better percent.