Hi Vaibhav, if you just want to prepare for IIT JEE mains only then NCERT is sufficient and there are around 18-20 name reaction given in it. So please focus on them only and mechanisms are not that important for IIT JEE Mains.
Diels Alder, Friedal Crafts reaction, Hofmann Rearrangement, Aldol condensation, Allylic rearrangement, Birch Reduction (explains the mechanism with Li and liq ammonia), Hell Volhard Zelinsky, Perkin, Pinacol Pinacolone, Riemer - Tiemann, Sandmeyer, Wolff - Kishner. The above reactions are the most important reactions. Apart from that you need to have an overall idea of other reactions.